Generally, the files in the System storage can be anything that cannot be categorized into app, movie, picture, music or document, such as:
1. Essential macOS operating system files;
2. Time Machine snapshot backups;
3. System log files and cache;
4. Cache from Browsers, Mail, photos and the third-party apps;
5. Old macOS updates;
6. Trash data and junk files.
Sometimes your Mac builds up an unnecessary amount of System files due to a bug, but most of the time it’s a normal part of how the operating system works. However, you can follow the below methods to manually reduce system storage on your Mac:
1. Empty Trash
2. Manage Time Machine Backup Snapshots
3. Optimize your Storage
4. Remove iOS Backups
5. Delete Cache Files
6. Update macOS
Hope it helps!