This is a great course and exactly what I needed (for visualization of theater lights).
When manipulating chairs/desks/PCs, I found it quite difficult to select the right objects. Usually I ended up selecting floor/walls. (I guess there is no "disable selection" option in Unity like in Blender. Disabling the objects wasn't too convenient.)
It seemed much easier to create a few new empties for "groups". Then I could click on furniture items in the project window (until I identified the right one), and drag it to the right empty. It was easier to move/rotate the empties than all the pieces.
Yeah it can be a bit of a pain working with selecting objects when you have a large scene in Unity. I didn't do it in the course, but you can place objects on specific layers and disable those. The empty game objects is a similar approach as well.