Hey WildBill! (Cool name), yeah, after the Big Sur upgrade, I noticed the same, because all files that used to be stored on the root drive (Macintosh) can no longer be stored there, so are moved to the non-read-only partition (Macintosh Data) where all your data is stored.
I thought this was only a one time thing after installing Big Sur, but have noticed it again after a Big Sur update as well. So, although files can’t be stored there anymore, I suspect system files may be being placed there for whatever reason. Initially after installing Big Sur, I’d check for any personal data within the folders, but now, most likely it’s just un-needed system data. Won’t hurt to check though!
Here’s a reference if it’ll help:
If you see a Relocated Items folder on your Mac after upgrading macOS:
https://support.apple.com/guide/mac-help/a-relocated-items-folder-mac-upgrading-macos-mchl8ae423a3/mac
I was mystified as first as well when I saw it, hope that helps!