I'm trying to find a convincing way to describe the lives of people in a city where the concept of nature is completely hidden from them. Of course, the buyers of the resources and corporations that run the city are aware. I guess a more general way to ask what's a good approach to describing a huge conspiratorial cover up?
Imagine a large Metropolis. Everything in the city is artificial, either made of concrete, steel, bitumen, etc. Meaning there are no green spaces and no plants. All food and garbage disposal is strictly controlled or automated so that animals can't get their hands on anything to eat. Animals would have almost nowhere to live and nothing to eat. They would either die of starvation or move outside of the city.
Birds could be dealt with by shooting them down before they enter the city perimeter. The other solution would be to build a dome, but that's not cost efficient. Surviving birds would be said to carry diseases form the exterior and people would despise them.
The city would be surrounded by a wall. It doesn't have to be that tall, it's just to make sure that people can't get in contact with the ones outside. People would not be permitted to leave the city or could be convinced by their benevolent government that the exterior wards are dangerous. It could be because of deceases or a high crime rate. The surrounding wards extend past the horizon and people would have only little knowledge of what's going on there. Furthermore, they have no idea of what lies beyond the horizon outside of what they are being told by their leaders.
Communications with the exterior need to be tightly controlled or simply shot down for the common folks. It is better for them not to be exposed to the harshness of the exterior world. Who really wants to know of a distant civil war raging in Africa? People do not need to know that, it makes them sad/change the channel. Give them something cheerful like the Kardashian, and you keep them happy!
Where does the food come from? What are we eating exactly? In real life, we do not know. The package description simply tells me it was packaged in Canada and I can't tell what half of the ingredients are. Most of our food is processed and we barely know anything about it. In the movie Soylent Green, people eat the green thing that looks like food. It's the only thing available so they are not asking too many questions. Spoiler: It's made of human flesh and the population is not aware of this.
For food and everything else, it needs to enter the city as a manufactured/transformed product.
There would be a requirement of a degree of brainwashing for any chance of this to take affect. Regardless, the people who initially enter the city will know what they left behind and this will be passed down in stories and through photos and books and movies. It would require a societal purge of current forms of media and much would have to be destroyed. The only plausible way for this to work is if the city's initial residents were of two groups: the conspirators who are attempting to create this population and children (those young enough that they wouldn't know any better). The children would have to be raised in the isolation and would know nothing more (so long as none of the conspirators turned on the collective).
I would recommend you read The City of Ember (or watch the movie - but the books are always better). It's a children's book but it is centered on city of people who are sustained by a giant storeroom of goods and a generator (that's failing). As far as the citizens know, there is nothing outside Ember.
Well, you need a different explanation of where resources come from, and this needs to be consistent enough to allow all the people of the city to believe it. This is likely to have bad effects on science education in the city's schools, and thereby on the city's economy. Why does anyone want to do this, given that it's likely to create a society of people who can't cope with any other environment?
This is easily explainable.
Some reason has forced people underground:
It could be a human or natural catastrophe. Either global warming and global cooling (there have been times on Earth were Earth was completely frozen (Snowball Earth) or nearly always tropical (mesozoic) with CO2 levels ten times as high as now. The possibilities are endless: supernova, nuclear global war, supervolcanoes, environmental destruction. Whatever it is, it collapsed the food source and triggered mass extinction on the surface. Remember: Such events did occur and nature was always able to cope with adapted lifeforms. We are principially able now with nuclear power to produce food chemically (it need not to be Soylent Green). What now happened is that the condition on the surface is getting better (it could still be very inhospitable) and some incurable nosy people have found that out and uses that to their personal gain (getting normal food, gutting gear from the surface). The problem is: If there is more than enough space for people up there because almost all people died, providers may lose their privileged position once the cat escaped out of the bag. That is something...uncomfortable.
Another possibility: People are living voluntarily underground. Well, not so much voluntarily, but because they lack money. You are trash...erm underprivileged and the only applicable housing is...look at the nice shiny PVC seats. In effect you are not able to leave, so people accustomed to concrete, junk food and artificial light.
I'll assume roughly current technology and that the population is split to majority that is kept in the dark and a smaller but still large trusted population that handles resource extraction, construction, security, long distance transport and so on.
Just build large buildings that have no windows or balconies and are connected underground. In a city like this the untrusted would never see the outside, just the insides of the buildings and tunnels deemed safe for them. It would then be fairly simple to hide nature from them. You might need to make up cover stories for origin of resources, but most people are not really that interested in how other people do their jobs, so this shouldn't really be an issue. If the state strongly insisted that milk comes from milk flowers grown in hydroponics or is created synthetically from recycled plastic, why would anyone doubt it?
Modern cities already have extensive underground infrastructure, so that part would be simply making it more so.
Buildings would be more novel, but with modern LED technology making buildings without windows might actually be more efficient. No sunlight intruding inside and thick external walls would make temperatures easier to manage. And you could cover the entire exterior with solar panels and extract wind power from the corners of the building.
Not needing to give rooms access to natural lighting would also make space management much easier. I think buildings would essentially be huge flattened cubes with the interior filled with individual spaces stuffed in a space efficient manner.
Have you read Isaac Asimov's Robot trilogy? Especially The Caves of Steel. In those days, people live in Cities, which are surprisingly like what you describe here.
A City is a huge cave of steel, to put it simply. Each City is surrounded by tons of empty farmland that the people never see, as all agriculture is taken care of by robots. There is no natural light or vegetation inside, only artificial. Everything is connected by tube roads or "strips" (like high-speed people-movers), and everything is under one huge roof.
The average person has heard of the outside but would never venture near it, as they have lived their entire lives indoors and are literally afraid of sunlight. A few more generations and outdoors could become a myth among the populace. "Tons of 'trees'? Bodies of water the size of a City? Impossible -- the world is far too cramped to hold anything else."
Food is provided in cafeterias by the government. In the books the people do know where food comes from, more or less, but you can change things around. Your ruling powers can simply hand-wave it away as having come from a lab.
A bit of encouragement in this direction on the part of the government, and you have your setting.