Why other system take so much storage

3-6GB might be normal,

If it is much bigger then you can Restore to factory and restore a backup and any bogus Other will go.

It will take some hours.

Backup and restore 

(1)How to back up your iPhone, iPad, and iPod touch – Apple Support

Unlock your SIM PIN if it has one

(2) Restore your iPhone, iPad, or iPod to factory settings – Apple Support

(3) Restore your iPhone, iPad, or iPod touch from a backup

Hi,

The following resources should be able to further assist:

Resources

• How iOS and macOS report storage capacity
• If you need more space for an update on your iPhone, iPad, or iPod touch
• How to update to iOS 15 or iPadOS 15
• How to check the storage on your iPhone, iPad, and iPod touch
• Clear Other storage on your iPhone or iPad