A lot of devs use prebuilt maps from modders that import them via unity. But when I first started I couldn't find any that really fit how I imagined the cafe to be, so I started making my own in HS2 instead (with the exception of the train, the mall, and the town centre). That's why the first ones I made that appear in 0.1 (like home/school) are somewhat shitty, I was still getting the hang of it.
Making a map in HS2 basically involves placing each asset individually; so that's every wall, floor, ceiling, piece of furniture, tuft of grass etc.
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See the "Workspace" window on the right? That's a list of all placed assets (that I've sorted into folders). Including the back exterior that I've just made, I would guess the downstairs of the cafe is probably over a thousand placed assets at this point.