It doesn't list such things explicitly but building magical stuff is just an extension of basically all other non combat actions.
You get some plans, you take some time, you spend some money. If the DM feels the action is risky, you make a skill check. If all of the above is done you succeed.
So building a flying city would just be the GM assigning a cost and timeframe to it. He'd probably say you need some specific stuff to accomplish it, like an ancient tome explaining the process and some rare reagents so your wizard had an excuse to go explore The Fetid Tunnels of Doom or wherever.
So there aren't actually rules for it, or even a real mention of it, and in general for everything that isn't stabbing with a pointy stick it says "your GM will make up costs, special rules, times, materials, sidequests and everything else and maybe set a skill check for you - hope you have advantage on it".
That's fine, it doesn't need to have any of it as it has no ambitions beyond dungeon crawl combat, but "wing it with these vague guidelines" does not agree with "Yes. Yes. Yes. Yes. Yes. Yes. Maybe".