Hello mistakoch,
Thanks for choosing the Apple Support Communities. Here is an explanation of what is contained in System Data.
"System Data: 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. You can manage your data that falls outside the other categories using Finder or the 3rd party apps that created it."
More info on storage categories can be found here: See used and available storage space on your Mac - Apple Support
Also, here are some helpful steps to manage your storage.
"Free up storage space in other waysDelete unneeded media, such as music or movies or podcasts, which can use a lot of storage space.Delete unneeded files in the Downloads folder. You can open the Downloads folder from the Dock or by choosing Go > Downloads from the menu bar in the Finder.Delete unneeded email in the Mail app. To delete junk mail, open Mail and choose Mailbox > Erase Junk Mail. To delete mail that you've moved to the Trash, choose Mailbox > Erase Deleted Items.If your Mac is set up for multiple users, remove users that you no longer want to have access to your Mac, or ask them to follow the steps in this article after logging in to their user account. Also check the Users folder on Macintosh HD for any undeleted data belonging to previously removed users.Check the Users folder on Macintosh HD for any shared files that you no longer need to share.Move some of your files to external storage, such as iCloud Drive* or an external drive connected to your Mac.Uninstall apps that you no longer use. Compress files that you use infrequently."
Free up storage space on your Mac - Apple Support
We hope this helps.
Cheers!
