I would create a library of modular house/building pieces and of stuff to litter the streets with, sort of like a reusable LEGO set with which you can build many many things 😊. And I would definitely go with mostly trim sheets and tiling textures, especially if you go big scale (like open world).
These two YouTube playlists by game environment artist Tim Simpson should give you a good idea on how to approach your environment, as well as how a good understanding on how to use and create trim sheets:
https://m.youtube.com/playlist?list=PLBi3xvwvY3dncU0vlOi7gov3AFmXhRTOm
https://m.youtube.com/playlist?list=PLBi3xvwvY3dk09fNSd7jrnEscXS_6_1p2
Also this talk from this years blender conference might be helpful:
https://m.youtube.com/watch?v=TkTQhmNFRnY
Hope this helps!
