Mac storage basics · System Data and the storage categories
What does "Other Users & Shared" mean in Storage settings?
Short answer: Files in other user accounts plus the Macintosh HD › Users › Shared folder. Clean up from inside those accounts, or remove accounts you no longer need in Users & Groups.
It counts files belonging to other user accounts on the Mac and the shared Macintosh HD › Users › Shared folder. You can't see inside other people's accounts from yours — that's by design.
To reduce it, sign into the other account and clean up there, or (if an account is no longer needed) remove it in System Settings → Users & Groups, choosing whether to keep a copy of its home folder. Some installers also leave large files in Macintosh HD › Users › Shared.