There is a fix for specific root cause, caused by a large number of mdworker files. If you are upgrading from macOS High Sierra, Mojave, or Catalina to Big Sur your upgrade could fail if it matches a very particular condition. If folders in multiple directories in the location below /private/var/folders///C/com.apple.metadata.mdworker have more than 20,000 files your upgrade could fail. In some situations users have reported 500,000 – 2,000,000 files!!! Entire solution is here: https://mrmacintosh.com/macos-upgrade-to-big-sur-failed-stuck-progress-bar-fix-prevention/