Kitbash3d Interiors

Hello,

I am trying to add interiors to my kitbash3d buildings.
Is there a tutorial on how to do this?
I am also trying to figure out how to make the building doors usable. Some sort of cut out or something but I don’t want to get rid of the doors I want to use them.
I am sure this is a lot of work, but I don’t mind as I have time to fill lol!
Sorry if this is a duplicate question, I just can’t find anything when searching. Didn’t know what topic to put this under either.
If anyone has something or knows a tut i would appreciate a link and thank you in advance.