In my story I have the thief school/college which built a city, or if you asked others was built because of the city (It was so long ago nobody knows.), underneath the capital of Mafalia by about six hundred feet. The capital of Mafalia has a population of sixty million built in a city that winds through basically the grand canyon and allows for it to be stupidly high. This is important because the waste from the capital goes to the sewer system that our underground city takes care of. The school easily houses and trains about 182000 students on average. For the past twenty or so years it's all been bad years. Before that the lowest it had gotten in seventy years was 300,000 students with the average being 500,000 (They get classes of about 500 once a week and the classes live there for seven years.)
I already have some infrastructure set up in the form of I know that they have two main sources of income. One fertilizer, they house and maintain the union of sewage workers who take the large amount of waste and use the heat from it rotting to bake the bacteria out of more sewage in a never ending cycle. They also filter out clean water so that the giant underwater lake can remain clean enough for it be let out through various underground rivers and streams or pumped up to the city directly above. And the second is scamming all of the various countries by counting their students for each school and also donations from various crime syndicates who got their start there or non crime syndicates who believe in public education which Mafalia and almost all worlds don't provide. I've already planned the sewer but if you want to use it or notice some flaws then please point it out. (The sewer supplies baked nightsoil to farms outside the city for more money that is funneled back to the school with a lot of it going to the union of sewer workers.)
There's also large kitchen staffs that feed everyone in the city through communal dining areas, not everyone decides to eat there but enough do, that does most of their prep work on Huntsday while the actual staff does everything else. The last two important parts I have mapped out are the cleaning and paperwork crew. The paperwork internship does secretary work. Filing the hundreds of students complaints, teachers complaints, detention, material request forms, pay, expulsion, and even (If they stay on for at least six years) budget. They file it on actual paper and then can upload it onto quartz crystals. More on that later. The other is the cleaning crew who, you guessed it, clean. They wash dishes, clean rooms, clean classrooms, clean bathrooms, clean, clean, clean. Very important.
Why Huntsday matters?
For the first year students all of them are assigned roughly evenly to one of the four people. The union needs people to be interested in their work so exposing large amounts to it and seeing which ones sign up again next year works, while the cooks just need all of the hands because they are feeding the students, staff, and whatever union (It's part of the collective bargain and is also the reason that the union pays for tools as opposed to the people hiring them) people show up. The cleaning crew does deep cleaning floor by floor so that every Huntsday you are either doing the equivalent of easter cleaning or you're doing general maintenance or sometimes both. Secretary people take the longest training but they also require the least people because most of the forms simply need labeling and filing and looking at maps to hunt people down.
Students generally take internships all throughout their seven years but most quickly start to move away from the basic four and branch out to whatever it is they want to study. Internships are once every seven days and last for nine.
The students take mandatory field trips or, if they have a home, go home during breaks. There are also gardeners but that's run entirely by the teachers and students and the trees are carefully planted to look really cool. They use water elevators and levis for drip irrigation but unless you can come up with a feasible option they will import food. Even if they could grow enough food they would still import luxury foods as the college wants their students to be skilled at most things and a wide diet helps since they can't micromanage what they're students do. (Even if they could it's considered to be a time honored art to smuggle in alcohol as a planet wide prohibition has been going on for the past seven generations.)
Lighting, technology, and food requirements
The world runs off 'magic' which in reality is just science. Long ago the human race moved to the stars and then stuff happened and all that was left behind was 'magic' that's not really understood well. For example: Potions=Chemistry, Inorganic Potions=Inorganic Chemistry, Divination=Math, Magic Crystals=Computers that can affect things
So for tech near future, by upwards of forty years and theoretical stuff we haven't perfected, as long as it doesn't need to be understood to be replicated. However the more non modern tech the better.
Humans also genetically engineered themselves to be able to produce a lot more static electricity and also shoot lightning. This is so they can charge things with their hands and also because it helped them control the mechanics they used for spaceships with just their electric fields. So they can produce light for themself and you don't have to worry about ventilation except for air and the like. The cave cannot have large openings. If you can fit two horses side by side it's too large.
The electricity is also limited, because this isn't the most efficient way to go about it, and is needed for a lot of things. If you can do everything possible except for lighting without electricity then please do so. The crystal quartz readings/recordings do require electricity but nothing fancy other then a magic stone. Also, people an average require five times the amount of minerals per person then the average person because of the extra lightening stuff.
A gold coin is worth about two hundred US dollars of buying power and the school gets about three fifths of its money from the various countries and donors. They can tighten their belt and pull out of the emergency reserve but they'd rather not so rough plans of how they get more money would be good. They cannot collect taxes or tariffs. They can totally collect money off their students internships if most of the students don't notice.
What I want
The infrastructure my upside down city would need and how it would work
What I do not want: How my city was built or how the buildings would work. I'm planning on asking those separately.
Edit: To clarify, the thief school/college teaches all ranges of students above the age of six, basically anyone who doesn't need a babysitter so like two six year olds and only if they're enrolled with someone else who will unofficially take care of them, but most people leave with a bachelor degree so I sometimes refer to it as a college. Sorry about that. Also, the capital of Mafalia is not the underground city. I don't know the population of the underground city past the students and faculty (It's roughly a one to six faculty ratio when it comes to administrators and teachers). I don't know how many other people there would be. The capital of Mafalia, which is above the underground city, has sixty million people and the union takes care of the sewage which is why I mentioned it.
Another thing the thief school, or thief college (They're the same word in my world), is the official name. It trains things other than thieves, however, a lot of people go simply because of the free education, food, and housing. They train diplomats, farmers, mercenaries, lots of scholars and well. Pretty much everything. After all they have a lot of students. (Most scholars don't stay on as teachers. They go elsewhere with a legal diploma to get into other colleges.)