Apple’s final word on managing the “System Data” category:
System Data: Contains files not listed here. It includes system files like log files, caches, VM files, and runtime resources. Temporary files, fonts, app support files, and plug-ins are also included.
You can’t manage this category’s contents. macOS manages them, and the size varies based on your Mac’s state.
Users have control over the User Account Folder (Home Folder). All other areas are inaccessible.
It’s good computer practice to keep at least 20% to 25% of the total drive capacity empty to avoid unintended consequences.
If as you have reported, 80% of the drive is used, the computer is approaching the magically and eventual warning from the Operating System >> " This computer is running out of Space " or similar message
Purgeable Space is controlled by the operating system. When the system needs more empty space, it moves some purgeable space to empty space.
There’s no user action to hasten this transition. It can take days or longer.
The links below help identify what’s taking up space on the internal drive and provide ways to remove user-controlled data (Home Folder).
Rebuild the Spotlight index on your Mac.
Free up storage space on your Mac.
GrandPerspective
How to delete Time Machine snapshots on your Mac.
This often occurs if the Time Machine Drive isn’t attached to the computer and TM Backup is set to run on a schedule.
TM Backup makes snapshots on the internal drive until the Time Machine Drive is attached. Then, the snapshots are transferred to the external drive.
View APFS snapshots in Disk Utility on Mac
See used and available storage space on your Mac.
Locate backups of your iPhone, iPad, and iPod touch.
If you use a suite of Adobe applications on this computer, they may create large cache files that can be removed. However, the Adobe cache files will be recreated as the applications need them.
https://helpx.adobe.com/ca/premiere-pro/kb/clear-cache.html
The same clearing of System Cache files can be achieved by booting into Safe Mode. They will be recreated as the system requires.
