Is there a tutorial set for making custom doors and windows? The Building section is about using Build Mode, not for CC Build Mode objects and I'm just not finding anything on that kind of stuff.

I imagine the meshing part on this to be fairly simple, I'm just not sure on how TS3 reads the doors and windows as cutouts, so I'd be more comfortable working off a tutorial than trying to figure this out by trial and error.

There are no specific tutorials for windows/doors as far as I know, but the process is the same as objects so you can follow those tutorials. There is an old tutorial by Ignorant Bliss for TS2 windows and has a section on the wall mask (the window cut out) that is still relevant for TS3.