Big Sur moved "My Documents" folder. Where did it go?

There is (and has been) a Documents folder within your User folder - That is where all of my life is (actually within a sub folder of it) and that has never been touched by any upgrade. Have you checked to make sure the stuff wasn't maybe moved there? Partial screenshot of a Finder window:

However, if yours was on the desktop - where was its' home or base location? If it was in a System folder, it may have disappeared and become inaccessible since the OS and system files are now on a read only volume not accessible to anyone or anything.

What, exactly, is your "My Documents" folder? That isn't a default folder on macOS. Where was it before installing Big Sur?

Items located outside of your home folder will get moved into the /Users/Shared/Relocated Items folder because you can't save files in system areas anymore.

Did you enable iCloud during the upgrade? If so, look in your iDrive.

"My life's in there! " Always create a backup before doing a major upgrade.