One of the few accurate reporting areas in macOS is Disk Utilities
Most of the Other Reporting methods ( space Used versus Free ) depend on Spotlight which is good for finding files, application etc
Not so good for actual Spaced Used
System Data is a collection of everything Spotlight can not needly categorize into known categories
As for managing System Data ?
You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.
There is Purgeable Space and there is Empty Space.
They are not one and the same thing.
Purgeable Space which is Controlled by the Operating System. When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space
AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space
It can day or longer before this will occur.
You may consider from the follow links
View APFS snapshots in Disk Utility on Mac
Locate backups of your iPhone, iPad, and iPod touch - Apple Support
If there a Suite of Adobe Applications used on this computer ?
They may create some very large cache files that can be removed. Though, the Adobe cache files will be recreated as the Applications needs 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
