Rooms are drawn in the order they were added.

latest on top? any wy to reorder (other than deleting and redoing)? can different Levels be used?

Lastest added on top (but with transparency in the plan).
To reorder a room you have to cut-paste it.
Only rooms in the levels at the same elevation are drawn (and it depends on which level you selected).

In the 3D view, a room floor will cut room floors below it.

'below' meaning 'added after/later'?

No, it means "added before".