Clean up storage on iPhone 12 Pro

Read this support article for how to check the storage on your iPhone, iPad, and iPod touch

https://support.apple.com/HT201656

- General article on seeing what is taking up storage on an Apple mobile device and tips on how to clear some storage space.

Note there's different kinds of "storage". Click on this link to learn the difference between your device and iCloud storage, ways to manage your space, and upgrading to iCloud+ if you need more --> https://support.apple.com/HT206504 & a user-tip https://discussions.apple.com/docs/DOC-250010973

Using the link provided by Limnos, if System Data is bigger than 10GB then you need to 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.