System Data taking all drive space on iPhone XR

Straightforward task to remove it

Back up the phone, Erase all content & settings, Set up the phone again, restoring the backup.

Resources:

Back Up: How to back up your iPhone, iPad, and iPod touch - Apple Support

Erase & Restore: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Then add back credit cards, sign back into accounts etc.

More difficult task is finding out which third party app or game is filling it with junk

maybe

How to clear system data on iPhone - Apple Community

Looking at my iPhone Storage and the "System Data" is taking up 86.61 GB out of 256 GB of my data. This is absurd, does anyone know why this is so high for me and how I might be able to get this lowered? : r/ios

What is iPhone System Data and How to Clear It

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System Data taking all drive space on iPhone - Google-