Recommendations

Hello everyone. I recently started studying this program and I really liked it. But I ran into the problem that I simply do not know how to make my measurement in the form of a backstage (the same 0 level) I just don't know how to make a measurement where there will be rooms and corridors, and not some kind of island or void. So please help me with the measurement, or rather how to make it so that any level of backstage can be done