Mojave-'system' takes up 800gb, why?

You can’t directly manage the contents of this category. That is done by macOS. The category varies in size depending on the current state of your Mac.

What you can do…

• Empty the Trash in the Dock. 

• Empty the trash in the Photos app.

• Delete unused and unneeded application installers from your downloads folder and desktop.

• Reboot your Mac at least weekly.

• Transfer files that you don’t use daily to an external drive and then delete them from the startup drive and empty the trash. Files that take up the most room are movies, images and music.

• Force Time Machine to delete local snapshots:

About Time Machine local snapshots - Apple Support

• Use Disk Utility to delete APFS snapshots:

View APFS snapshots in Disk Utility on Mac - Apple Support 

This from Apple on the subject of storage management

Free up storage space on Mac - Apple Support

You can also use apps such as DaisyDisk from Software Ambience (also available in the App Store) or OmniDiskSweeper from Omni Labs.  (I’m unaffiliated with both.) Once you've located these larger user files, you can relocate them to another drive and/or delete them if they aren't needed.

You're welcome. 🙂