>>299879232
There's some free stuff on the unity asset store, search for modular buildings. You can also use e.g. the hl2 assets from hammer, there are guides on how to get those into unity. Download the unity demo

For blocking out levels, you can try ProBuilder, unity's official plugin for light mesh editing in blender. It's okay for simple stuff like stairs, doors, ramps, etc. You can also try out sabreCSG for a quake-like level prototyper.