I have iPhone iOS 15.2 and I can’t clear the system data which taking a lot storage! Help!

Back up your phone, and restore it:

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

The article linked at the top of this page, doesn't apply to your iPhone. Also, your iPhone is way out of date and needs to be updated to iOS 15.7.7.

When Other becomes too large, there is often some corrupted data and the only real way to clear it is to backup your iPhone, then restore your iPhone from backup. Since your iPhone needs to be updated, you should use your computer to do the update.

These articles will tell you how to backup/restore/update

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

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

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

That's not going to help them much. They have an old iPhone, with an out of date iOS, which has been collecting system data for a LONG time and it needs to be backed up and restored to clear all the years of system data stored in other.

I totally agree with that. I'm just saying if someone has something uploading or downloading from iCloud it can sometimes go into system or other storage for a bit and make it quite large. But I agree the device definitely needs restored. My comment was just an addition to the other information already provided.

Do you have anything uploading to iCloud? This can go into the system data area until it's uploaded or downloaded to or from iCloud.