In theory, at least, a macOS Big Sur update cannot affect user data because the system data is stored in its own sealed volume and only a snapshot of that volume is mounted and mounted read-only. The data volume, which contains user data, is not touched and is not even mounted during updates.
As for APFS data corruption - if it happens again, I would suggest running fsck_apfs. Apparently APFS data corruption is fairly common according to
https://bombich.com/kb/ccc5/coping-errors-caused-apfs-filesystem-corruptionI've not suffered it <touches wooden desk/> myself. I have suffered HFS+ data corruption of Time Machine volumes, though after hours of running every option to fsck_hfs, I recovered everything (amazingly).