How would I make 3D buildings?

How much detail usually goes into them? Is there any use or need for this information, or the interior layouts of public buildings?

As much detail as you want. Check out Denver on F4Map if you want to see some very fine detail.

You may also want to have a look to Den Hoorn, Zuid-Holland, Netherlands, especially when it comes to terraced houses.

Or look for Kremlin, Moscow. I really like this example :slight_smile: