You either have very few interesting interiors or a lot of generic ones. So how are they going to tackle this? I don't think there's an open world game that doesn't suffer from this when it comes to interiors. You also need many floors and rooms to be 'useless' since you don't need everything in a virtual world to have a purpose, otherwise it will feel fake, as if it was made for the player in mind. Unique hand crafted floors and rooms will be time consuming though and will probably increase the file size by a lot. Copying-pasting hand crafted rooms will also make the buildings boring to explore after a while. They will look generic and won't have many differences between them other than furniture being moved around and maybe having a different texture. The problem with this is that the randomly generated floors will not be interesting.

It's not hard to make a building with many floors and let a program randomly generate the floors/rooms and then only hand craft the few rooms you want for a mission.

I wonder how much of this is hand crafted though? And how much will be copied-pasted?

Gta 4 all enterable buildings map