I can't choose files to back up

With Kopia-UI I can select only the folders in the choice of items to save. I can’t choose files. It’s normal ?

Yes, that’s actually by design. In Kopia you define sources and those only anchor to folders. There are other ways, but those depend on you host OS and code/script skills.

Yes, there’s way. Exclude everything and then include whatever you want.

#exclude all
#include filters

This from Windows. Path are relative. Also, even in the Windows, filters are case sensitive. Also, also this way of filtering has its own quirks, like producing empty non-include folders.