System Data Taking Up Significant Disk Space

System Data usually blows up when your iPhone is holding onto cached files, failed downloads, message attachments, or ost commonly local snapshots and temporary system data that iOS hasn’t cleared yet.

The quickest fix is simple is restart the phone first. Then check Settings > General > iPhone Storage and scroll down to large apps like Safari, Messages, and social media, clearing their caches can instantly free space.

If you use iCloud backups, toggle your backup off and back on to force iOS to clean leftover temporary files. And if System Data is still massive, back up your phone to a Mac or PC and then perform an Offload + Reinstall of big apps like Instagram or WhatsApp, they’re often the ones hoarding hidden data.

This usually drops System Data back to normal without needing a full wipe.

what is now called system used to be called 'other'.

This article is still pertinent:

What is “Other” storage on a Mac, and how… - Apple Community

if you have configured Time Machine, you can connect your backup drive and have it make an updated disk-based backup. This will cause Time Machine to release the snapshots (lists of file references) it has been accumulating on the boot drive, waiting for the physical backup drive to become available.