Big Sur is a disaster. Some older Word files show as zero bytes. pdf's not opening in preview.

Big Sur was released 9 months ago. If the kinds of problems you are experiencing were common these boards would have blown up long ago. Of course it's possible assuming you have an up-to-date version of Word or an equivalent. If you have files showing zero bytes then you have a local issue, and one that is easiest solved by simply restoring them from the back up you made prior to upgrading - you did make a back up, right? There is a song search in Music, make sure you've selected the correct tab:

If one is that unhappy with Big Sur - it is best work throughout the issues.

If that is not to your liking - Revert your Mac to a previous macOS version provided there is a Time Machine Backup prior to the Big Sur Upgrade.

You are not looking close enough....

Also terrible downgrade from I-Tunes to Music. No proper song search function for Library.

Going backwards Apple..This Mac Book 'Pro' is really a leisure machine increasingly useless for work.....

You can easily search your library in Music.

Just type into the search box and choose Library

If iTune Search function is that important and a really deal breaker - revert as per previously supplied Link.