Start with the storage view
System Data is a macOS category, not one folder you can safely delete. It can include caches, logs, app support files and local system-managed data. The total can change while macOS updates its measurements.
Open the storage view in macOS settings first. The location and labels vary by macOS version. Check whether apps, documents or downloads explain the growth before looking at System Data.
Look at paths, not just a total
In CleanPanda, use Storage to inspect the breakdown and Disk map to follow large folders. Read the full path and the explanation for each finding. Scanning lists files before anything moves.
Give a long check time to finish. A size marked with “+” is incomplete; an unreadable folder is not a zero-byte folder. Do not treat an estimate as a promise of recoverable space.
Keep the removal narrow
Review caches that the owning app can rebuild. Keep app support files, backups and files you do not recognise until you know what uses them. Do not manually empty System or Library folders as a shortcut.
Move selected items to the Trash and check that your apps still work. Undo is available while the items remain there. Disk space is not permanently recovered until the Trash is emptied; emptying it removes that way back.