This is a paid asset, but now you can download SciFi Platform City Free.
Detail this asset from Unity Store: Original Link
SciFi Platform City v0.8
With SciFi Platform City you get a tool to create futuristic cities with different levels in a modular workflow. It comes with near 140 prefabs including different blocks to create exteriors & interiors and also elevations & floating platforms to make it spicy. Just add different details from electronics, furniture, etc. Blocks are made to fit among them with a few clicks. You can also use it as FPS or Top/Down environment since rooms are made of separate pieces so you can remove roofs whenever you want. Finally includes high quality materials that allow a high level of customization for the final lookup.
Check out [ASSET INFO] link above for extended info.