system data using 126gb of data half my storage

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

The final word from Apple on Managing the " Other " Category

Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. 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.

And apparently Apple has a new way of hiding files & more than a few find out the only way is to Backup, then Erase the Drive!?

Terminal code to clean DocumentRevisionsfolder…

https://apple.stackexchange.com/questions/313102/what-will-occur-if-the-documentrevisions-v100-folder-is-deleted

macos - What will occur if the .DocumentRevisions-V100 folder is deleted? - Ask Different (stackexchange.com)

https://discussions.apple.com/thread/253231342?answerId=256043117022#256043117022

System Memory OS 10.12.6 Sierra - Apple Community

Look for iOS backups…

/Users/YourUserName/Library/Application Support/MobileSync/Backup

OmniDiskSweeper shows you the files on your drive, largest to smallest, and lets you quickly Trash or open them.

https://www.omnigroup.com/more/

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates. 

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

tmutil deletelocalsnapshots /  # deletes all the snapshots

Freeing up disk space - Time Machine snapshots

Applications -> Utilities -> Terminal

tmutil listlocalsnapshots /

Snapshots for volume group containing disk /:

com.apple.TimeMachine.2021-04-01-142742.local

com.apple.TimeMachine.2021-04-01-163342.local

com.apple.TimeMachine.2021-04-01-174425.local

com.apple.TimeMachine.2021-04-01-192243.local

com.apple.TimeMachine.2021-04-01-203820.local

com.apple.TimeMachine.2021-04-01-222814.local

com.apple.TimeMachine.2021-04-01-232930.local

com.apple.TimeMachine.2021-04-02-002823.local

com.apple.TimeMachine.2021-04-02-012748.local

tmutil deletelocalsnapshots 2021-04-01-142742 # deletes 1 snapshot

tmutil deletelocalsnapshots /   # deletes all the snapshots

Also see:

    diskutil apfs listSnapshots /

Then look for large files in you home folder tree

du -hx $HOME | sort -h