Since the starting conditions for colonists vastly improved, I am going to write another answer. Basic assumptions:
- colonists can grow crops in local soil;
- local flora and fauna are not toxic to colonists;
- the climate of the planet is similar to Earth;
- colonists, except spacecraft crew, are not trained for space travel and exploration;
- colonists are random people from a developed world (the latter is important);
- colonists brought vast knowledge databases with them, including technical blueprints;
- there is no magic genetic replication/synthesis machine;
- the landing was successful, the majority of craft is intact, but cannot go back to space;
- the land life developed insects and worms (they are necessary for plant pollination and modern soil development);
- there are no birds, reptiles, or mammals;
- seas are full of life, including early fishes
Skip the Middle Ages
Your colonists cannot and should not go back to the Middle Ages technology since it relies heavily on animals and wood. You have neither.
With 40-75 thousand people (although a more specific number would be better) and functioning landing craft you can go straight to electricity-powered technologies.
You can use your landing ships as shelter and power generator for the first couple of months. Your craft cannot be power-dead since you mention limited manufacturing capabilities and map/resources surveys.
Using one of the vessels build a hydroelectric power plant. A nearby fast-flow river or better a waterfall would be ideal. You are not up to building dams unless your limited supplies include some construction equipment and materials.
Immediate Population Census and Re-education Centres
It sounds like your colonists are refugees. While some of them were, probably, selected for this mission, some just managed to hop in. Conduct a population survey (better while still in transit to the planet):
- standard demographics (age, sex, marital status, etc.);
- education and practical experience (basically, education and work history plus hobbies);
- medical history (even though your population is big enough to avoid inbreeding and other unpleasant consequences of the population bottleneck, you have to be prepared to deal with a heart attack of your senior mechanic).
Establish a system of triage to divide people according to immediate survival needs:
- useful skills (engineering, city planning, medicine, electricity, biology, geology, agriculture, etc.);
- semi-useful skills (psychology, sociology, law, computer programming, etc.);
- useless skills (finance managing, banking, hospitality services, etc.).
Developed countries have service economies, so the majority of your colonists will have skills not so useful for survival or rebuilding the civilisation. Do not dismiss those skills, though. A hospitality services specialist might not know about catching fish, but they will have insights for setting up comfortable accommodations at a later time.
Setup education and apprenticeship programmes. Some can be started back on a ship, some will have to wait till landing. Teach as many people as possible to produce food. Train electricians and electrical engineers. Start training children and teenagers as research aids for scientists (science involves a lot of data sorting, keeping, and entering; your scientists would appreciate all help they can get with this).
Ration your food from the very beginning. Lock it if needed. After landing start food production immediately. Amadeus has some great suggestions for fishing. Starting agriculture might be challenging, but it is absolutely necessary if you want to rebuild your civilisation.
You can go with conventional farming using crop rotation techniques, but it is a backbreaking work even with animals. You will also depend on weather a lot.
Since you have a lot of knowledge and at least some technology available, you might be better off repurposing some of your equipment for vertical and greenhouse farming and algae breeding. Hydroponics and aeroponics might be within your grasp depending on your original spaceship. If it was used there, you might scavenge the system before landing. If you cannot afford those, go with soil but in a greenhouse use materials from your ship. This will give you better yields.
No matter what kind of farming you decide to do, make sure to start using electricity as soon as possible: lights, machines, heating, etc. If you use the medieval farming approach you might end with almost 90% of population farming. And you need those people to rebuild the civilisation.
Electricity and algae fuels are your best options. Start figuring out how to produce them ASAP. Electricity might be easier since any biofuel requires an industry to refine it.
When deciding how to recycle your ships give priority to building an electric grid. It will help to rebuild the industry.
Mining, smithing, and such should be your #3 on a list of priorities. You have to replace tools and manufacture new ones. Moreover, since your world does not have wood or bone and you cannot produce plastics, metals will be your main material for years to come.
You will not be able to power your furnaces with wood. You might not have coal. But you can and should use electricity. Start small, grow bigger only when you need to increase production or have electricity surplus.
Another important industry that has to be established ASAP is glass manufacturing. Glass is great for your greenhouses. It is also important for keeping science going.
Pottery and Clay
Without wood and plastics, clay will be your best friend. You can make bricks to build buildings (but I believe concrete is a much better option). But most importantly, you can make pottery to store your food.
Busy your colonists with the invention of concrete. It is a very versatile building material. You can construct roads, dwellings, supports, etc. Concrete manufacturing will be the most important step in building modern infrastructure.
You might consider working on some glazes and paints. Plain concrete is boring and concrete cities are bad for mental health. Splashes of colour and variety of forms in city architecture help to decrease depression and suicide rates. They also provide better stimulation for little ones.
Did you bring tree seeds? If you did, it might be a good idea to start growing a forest. It will not be useful for your colonists, but their descendants centuries later might appreciate your effort.
You might not be able to find anything that can be used as timber in your world. Reeds and bamboo-like species still can be used for building. Maybe your world has them. You can start selection work on bamboos so they have more wood-like properties. If you have access to DNA sequencing equipment selection will go much faster.
Your population is quite big. You cannot expect that it will just function. You will have to establish some form of government, law, and education systems right from the beginning.
If the majority are civilians a military or totalitarian/authoritarian approach might not work. Although, it would be the most effective for getting things done. Perhaps, you can do with representative democracy, but keep the bureaucracy to a minimum. You need all hands you can get to do the manual work, at least in the first decade.
You will also need laws. There will be crime. You have to decide how to deal with it. Harsh punishments reduce petty crimes but increase violent crime (especially murders). Can you afford jails? Are you going to adopt forced labour? What is your stance on capital punishment?
An educational system is very important for rebuilding a civilisation. You can try to recreate our system or go with other approaches. I think that compulsory early education in combination with apprenticeships could be a good approach in your setting. However, make sure that you do not have guilds. Children should be free to choose their occupations and information must be free.
Your population should be encouraged to grow. Yet, at the same time, you have to have 100% employment. Set up daycare centres, involve elders, encourage parents to bring children to work. If you are keeping a traditional family structure, make sure that fathers are active participants in child-rearing. You might also consider 'communalizing' house chores. That's where your hospitality service specialists come in handy. Set up communal kitchens, cleaning and laundry chores rotation, etc. Make community services mandatory for all men and women regardless of their professions, age, and social standing. Involve children from an early age.
You will have insurgents. There will be people who decide that they can do better on their own. Let small groups go, but do not allow your population to split into 2-3 big camps. You need all those people to work together. It might actually be an interesting plot development if a significant chunk of the population (about 10 000 people) decides to resettle elsewhere and truly start a medieval society. Then you can have two different civilisations on the same planet. You can even purge the records of the Big Insurgency, so once they meet they are very surprised.
You will have to start with socialism and communal property. There is no other way because you will be working on huge projects from the start. You cannot allow economic interests and private property to interfere with industrialisation.
Your colony might evolve into something else later. But you should keep it socialistic as long as possible, at least till you achieve about 20th century self-sustainable level of industry and population of millions. Your human resources are very limited. Every individual should have the best chance at realising their potential. They should have free access to knowledge, medicine, and daily necessities. Once you are out of rations you might consider universal basic income.
Arts, Crafts and Culture
Your colony will develop a unique culture. Most likely it will be a mix of cultures of original colonists adapted to current conditions. In order to specify features of this new culture you need to answer several questions:
- Does the majority of your colonists belong to the same culture? (People from the same culture have easier time working together, but are more constrained when it comes to finding original solutions to existing problems)
- If it was one culture, which one was it? (US culture is very different from Chinese; Chinese colonists will be more likely to setup a system that benefits community first, while Americans might demand more privacy and personal freedoms)
- Are there any minorities of any kind and how are they treated?
- Is there some vision of an 'ideal' society?
- Is there any specific social engineering?
All of these will greatly affect attitudes, expectations, and cultural practices of your people. After a century on this planet, the society will be different and many cultural references will become obsolete. In a world with no forests 'Don't see wood behind the trees' makes no sense. You need to come up with new references and sayings.
Arts and crafts will not disappear completely, but likely to experience a decline. Woodworking, woodcarving, and such will disappear completely. Painting, pottery, stone sculpture, metalwork will flourish.
The architecture will start as very utilitarian. However, as the colony becomes more and more self-sufficient and even wealthy buildings can become more elaborate and richly decorated.
Due to the lack of resources, people might switch back to living on a floor as in Japan. Without wood, it will be hard to make tables and chairs. Rugs and hammocks might replace furniture we are used to now.
You will have to come up with new musical instruments if you want music. Although, your composers can use synthesised Earthen instruments if they retain access to knowledge databases.
Fashions will change considerably due to the lack of traditional materials. In this new world, you will have no wool, leather, silk, or contemporary synthetic fibres. But you might find some interesting replacements (spider silk? fish skin?).
There might be some customs and traditions associated with landing and rebuilding. You can also have some remnants of old world like Christmass.
Expect a lot of discoveries in fields of marine biology and various bacteria. Your colonists will be in search of replacements for plastics and fuels. They will be researching bacteria a lot since we know that they can do a lot of useful things.
Also, think about all the small things that the first generation of colonists will be missing from Earth. I bet they will come up with some substitutes for alcohol or/and recreational drugs within the first year.