macOS Big Sur Users folder issue
First thing - don't try to fix anything.
Check when your last back up was done. If you don't have a backup - stop and back up this Mac to a blank external drive (ideally) using time machine and be sure you have a good backup before proceeding.
Now write down some details:

When did you start the update.
What was the previous version ( softwareupdate --history should tell you all the recent updates on the Mac)
What is the state of your storage ( diskutil list is a good indicator to share with someone technical )

Decide if you want to use Apple, someone local to work with, or trust us randos on the internet to know what your setup is and hopefully give good advice. The problem with us, is no matter how smart, honest and well intentioned we are - we only see your word and if you are mistaken, we may give you bad advice. Also, we could just be evil but hopefully if you are patient - votes will help see if we're all evil together or someone has good advice.