System Data & Time Machine Snapshots

Open Disk Utility.

Select the Data volume in the sidebar.

The Time Machine snapshots should be displayed.

If they are not, choose View->Show APFS Snapshots.

You can then select any snapshots and delete them right there by pressing the "-" button.

Yes, Time Machine Backup Snapshots can accumulate if TM Backup is on a Schedule and the destination external drive has not been attached for an period of time

TM Backup will make snapshot(s) and retain them until the destination drive is attached

In theory, once it is attached, it should transfer the snapshots but sometimes it may not

View APFS snapshots in Disk Utility on Mac

Depending on your success ( hopefully so ) visa vie Snapshots build ups was the root cause of System Data growing to large

If this has not been resolved ?

Reducing System/Volume/Data is a common question. 

1 -  System data taking too much in MacOS Sono… - Apple Community

2 - Time Machine Local Snapshot won't delete - Apple Community

3 - Over 60% storage blocked by System Data - Apple Community

How to free up ‘System Data’ and other storage on your Mac from a fellow colleague  @ neuroanatomist

Use another application to see where space is being used  Storeograph  on the Apple Apps Store 

Suggest getting an External SSD Drive and start moving your Pictures, Videos, Music and any other large files you have control over, OFF the Internal drive and Onto the External

Understanding iCloud Drive from a well written User Tip from @ Richard.Taylor

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.

Thank you too for the followups

About to only additional information that maybe useful is related to Purgeable Space

Not often discussed except when System Data issues are involved

From another contributor @etresoft regarding Free Space and Available Space 

Free vs available disk space huge differe… - Apple Community

Quote >>  “ The "available" storage is the amount of used storage that the operating system could automatically delete if it felt that it was really necessary.

The "free" storage is the amount that you can actually use for something.

There are system processes that run in the background and automatically delete some of the "available" storage and convert it to "free". If you completely run out of storage, then those system processes will try a little harder.

When you "delete" files you are just hinting to the operating system that you don't need those files anymore.

The operating system will eventually remove them, but on its own schedule.

Certain tools will allow you to force the issue and manually clean up some of this storage and manually delete local snapshots. But that is only temporary. "  << End Quote 

Purgeable Space is controlled by the Operating System and not the user 

Get detailed information about a disk in Disk Utility on Mac - Apple Support (CA)