As mentioned in TidBITS #1587 (https://tidbits.com/2021/11/03/macos-12-monterey-upgrade-issues/) (scroll down to the "Unrelated" section), if you are preparing to move to Big Sur, there is a problem that may cause you problems. However, there is a fix, using Terminal, that can determine if you might have that problem and allow you to avoid it.
BTW, if you are not running on an SSD, the command to find how many metadata files you have may take several minutes. Be patient. You will likely be starring at a Terminal screen with nothing apparently happening. :p
See if you have the problems cause and correct it proceed to Section 2, Step 1:
macOS Upgrade to Big Sur Failed!
Stuck Progress Bar fix & Prevention! (https://mrmacintosh.com/macos-upgrade-to-big-sur-failed-stuck-progress-bar-fix-prevention/)
Copy the code from the Mr. Macintosh site (Section 2, Step 1) into a Terminal window to see if you have massive numbers of metadata files. To assure a less problematic upgrade to Big Sur, copy the two different '-delete' commands (Section 3, Step 1) and paste into Terminal (one at a time). To see if the number of metadata files have been made more manageable, tun that first command (Section 2, Step 1) again: 'find... metadata.mdworker -type d | wc -1 (that's a numeral one, not a lower-case letter)
I believe there is more info on these files at MacRumors (https://forums.macrumors.com/threads/spotlight-mdworker-shared-file-flooding-system-log-removal-stops-search-index-from-working-properly.2248163/page-3?post=30206048#post-30206048).
Images below show the output from my wife's MBP.