Hello TanNguyenVn,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with the storage used by System on your iPhone. Congratulations on your new iPhone! To start, check out the information here about the storage in your iPhone, the different categories, and what they mean: How to check the storage on your iPhone, iPad, and iPod touch
"System: Space taken by the operating system. This can vary based on your device and model."
If you haven't already, you might try updating your device to iOS 15.2 to see if this makes a difference in storage reporting. Be sure to back up first: Update your iPhone, iPad, or iPod touch
If that doesn't make a difference, use this link to Get Support directly from Apple.
We hope this helps.
Cheers!