Executables only: .exe and .com basically, regardless of where they are. It scans all fixed disks.
It’s not a backup system, storing copies. Just some information of the executable are kept. The last three versions of each file info is stored.
No. This is an automated processes when avast tries to repair infected executables. You can’t manually add a file into VRDB. Again, it’s not a backup feature.
Pass this one as you want an official answer…