Mysterious folders that appeared in Documents on upgrade to Big Sur
All of those folders could easily come from iCloud documents sync. You are correct to be cautious deleting them. I would be sure iCloud is signed out (or at least you don’t enable document sync) before trying to delete these.
We can’t really look at your iCloud settings, so a chat or call with Apple support would be prudent if you don’t see right away what happened to be sure you do not delete iCloud data.