freeing up storage space & system data

Free up storage space on Mac - Apple Support

"The category named System Data (or Other) is a general category that measures the storage space used by all Apple and third-party files that don’t belong to any more specific category."

For instance, even though there is a category called "Photos," on my Mac System Data includes MANY large Photos Libraries.

Apple's "accounting" for usage of disk space through the "Storage" button leaves something to be desired. On one of my Macs it says calculating and never reaches a final result.

One of my Macs shows 90 GB for System Data; my wife's Mac shows about 25 GB for System Data. Because Apple says it can include "third party files," it can really include almost anything. In some cases, people have reported that System Data even includes some attachments from email files.

I suggest that you obtain a good commercial tool such as DaisyDisk that will (when you run it as administrator) tell you where EVERYTHING is on your computer and how much space it is taking up.

See also this informative article: How to free up ‘System Data’ and other st… - Apple Community

Reducing System/Volume/Data is a common question. 

Refer to the multitude of same / similar questions specific to your issue

 Search - Apple Community

There are  two effective ways to remedy this issue:

1. Quick Fix Actions:

   For Apple Silicon computers, use Disk Utility to erase a Mac.

For Apple Intel computers, use Disk Utility to erase an Intel-based Mac, then reinstall macOS.

Always make a Time Machine backup before proceeding.

   Migrate only the user account, not the entire system.

   Reinstall only the necessary applications from the Apple App Store or directly from the developers.

2 - Generally

When the user discovers this issue, it’s likely because the computer’s internal drive capacity is small, such as 256 GB .

Unfortunately, the user’s storage needs may have increased since the computer was purchased.

To future-proof the computer, consider spending extra money upfront on a larger drive capacity and adding more unified RAM.