I was wondering how hard it would be to make a city, say the size of los santos from GTA 5 and have every single building have interiors on every floor. is this possible? how much ram would it require? is there any tricks that could make this work?
You could make the interiors visible only when the player gets nearby, that should help a lot.
Technically it shouldn’t be that difficult. You would make heavy use of level streaming. For example only load the interior of a house when you get close to it. For large buildings you can stream each floor separately. Depending on the detail of the city you can do the same with the outside view of city regions.
The hard part would be to create and fill all those houses with interesting content.
^exactly, the things you would have to do to make something like that function would mean that you wouldn’t have much variety so there’s not much point in doing it
You couldn’t probably script something to populate the rooms/floors with a pre-determined library of parts when the first player enters the “area” and save the layout once that’s performed. You could make each room fairly unique but it still wouldn’t look like an artist had been in a decorated it.
Could be very resource intensive.
You’d still probably want to use interior mapping for windows when viewing a large building from outside.
Managing LODs, streaming, and memory load, is going to be a nightmare 
But it’s possible right? how much more feasible is a city a quarter the size of Los Santos, with tall buildings but no skyscrapers? Or like a small town or something?
At a very small scale you would do it manually, but anything beyond that would have to develop a complex system
Now that I think about it, State of Decay did something similar. The City of Marshall is about the size I could make it, I just want the basic stuff that you would find in any city. Of course that game was made with a different engine and in 3rd person but I dont think it would be difficult to do something similar in unreal.
While I applaud your vision of doing something so complex, unless the games core mechanics actually involve the use of building interiors in a meaningful way I see no reason to do it that extensively. Now I did get the idea the other day that a game loosely based off the Judge Dredd comic theme would be interesting. Large scale slum style skyscrapers with thousands of rooms that could be claimed by players and customized.
Make a ciity like Bath in England. If you look at the royal crescent in Bath it’s very easy to make in UE4. lots of formal repeats. With all the same floor plan. Interiors are very similar. The society was very conformist. You could have a range of wallpapers and furniture and swap them around.
As has been said, it won’t be reasonable to manually model and populate interiors on such a large scale. So you’d essentially need some kind of procedural approach. That way you take away the need for ridiculous amounts of level and content design, and also reduce runtime resource problems, as you generate parts of the city on demand as the player(s) moves around.
It would of course be a pretty big undertaking to write a system like that. I’m currently working on a potential marketplace plugin for building generation, but it would only form a small piece of a solution to what you want.