100GB of system data
Solution 1:
If possible, try deleting an app or two (optimally, one with some data stored on the iPhone) and see if this changes. If it goes up, this could be a "known" bug, in which case the only solution I'm aware of is, sadly, the nuclear option.... wipe the iPhone from Settings, and restore from backup (make sure to backup first!).
Solution 2:
After saving some space to download the update and installing it, the bug has been fixed.
Only 9GB are used in this category.