Salutations Veloman345,
You’ll find some information on what is held in System Data in the following article - How to check the storage on your iPhone, iPad, and iPod touch You may see a section for iOS and then one for System Data. System Data is going to include Siri voices, fonts, dictionaries, non-removable logs and caches, Spotlight index, and system data, such as Keychain and CloudKit Database, etc. This data isn’t removable by the operating system.
Your device will automatically delete the cached files it can and temporary files when it needs more space. Are you receiving notifications that your device is almost out of space?
Also, with a new device set up, restore, iCloud sign in, that System Data may temporarily be increased while the sync is finalizing.
Thanks.