What is system data? Because that’s what’s taking most of my space

See the last 2 items in the 'Content categories' section of:

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

"Other: Non-removable mobile assets, like Siri voices, fonts, dictionaries, non-removable logs and caches, Spotlight index, and system data, such as Keychain and CloudKit Database. Cached files can't be deleted by the system.

System: Space taken by the operating system. This can vary based on your device and model."

That is explained in Settings/General/iPhone Storage, underneath the value for System Data. To see if it is cached data, open Safari, tap on the overlapping squares at the bottom right, and see if you have a lot of pages open. If you do, close them, then restart your phone to see if that reduces System Data.

Sometimes system data gets stale and doesn’t recover unused space; usually that can be fixed by backing up your phone, then restoring the backup.

1. How to back up your iPhone, iPad, and iPod touch - Apple Support
2. Restore your iPhone, iPad, or iPod to factory settings - Apple Support - follow the instructions to restore using your computer, do not using Settings/General/Reset (which is not included in those instructions)
3. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support