Once I import a file, and I corrupt it (having anki synchronisation turned off) the deck is still fully functional. So where is a copy of the imported file stored? What folder in macos or windows?
As explained in the relevant section of the manual, all Anki's files are stored at the following places:
%APPDATA%\Anki2~/Library/Application Support/Anki2~/.local/share/Anki2However, there are some gotchas. If you need more details, see the link.
Besides that, beware that decks are not stored as individual files: the whole Anki collection is a single database, so you can't copy decks around. However, you can still export decks from within Anki.