Where did all my files and apps go after uploading Big Sur?!!

Applications did not go anywhere. They are in /Applications, as usual.

Some of your files may have been moved to iCloud, if you chose to (unintentionally, it seems).

There is an option to put the Documents and Desktop folders in iCloud.

Open a Finder window, click iCloud Drive in the sidebar, and see if there are folders there called "Documents" and "Desktop".

Other stuff, like Pictures, Music, etc., will be exactly where it was before.

I'm not sure where apps would go. From where did they go?

For your files, are they in iCloud Drive. Many people just click through the startup assistant and end up enabling Desktop & Documents in iCloud Drive.

Another possibility is if you stored your files at the root level of the hard drive, they may be in the Relocated Items folder in /Users/Shared.

Just upgraded to Big Sur 11.6.

My playlists no longer come up in Music

Where did they go

My playlists no longer come up in Music

Where did they go

No idea. I'm not sure how it is related to missing files.

Suggest, for better Assistance, to Open A New Question instead of piggybacking on another persons' question.