System Data over 70GB with iOS 15

That’s not good, and probably means there’s storage corruption. It can usually be fixed by backing up your phone, resetting it to factory settings, 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 (restore using your computer, not “erase all content & Settings”)
3. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

That can indicate either storage corruption on your PC or an old backup on the PC that is damaged. In iTunes Preferences, Devices panel, see if there is an old backup. If there is click on it, then click Delete followed by OK. Then wait a minute for the delete process to complete.

I see occasional reports of this issue. It can be caused by an interrupted sync, and I’ve read that there was a bug in WhatsApp that can cause it by expanding the cache to use all space. I think the WhatsApp issue has been fixed, but the damage caused by it doesn’t go away by itself.