Why is System Data taking up 100 GB on my iPhone?

Start with a restart and make sure iOS is up to date, then check Settings, General, iPhone Storage again. System Data can include caches, logs, downloaded voices and other files that don't fit neatly into an app category. Some variation is normal, but 100 GB is a lot.

There's no single button to clear all of it. The storage display may also take some time to recalculate, so give it a bit before deciding whether anything changed.

If there's an iOS update available, try installing it through a Mac or PC. That can be useful when the phone doesn't have enough space to update in Settings, and it may reduce the system storage. It isn't a guaranteed fix, though. The data can grow again afterward.

Back up first, and choose the update option rather than restore if you want to avoid erasing the phone.

Before going anywhere near a reset, check apps that store lots of temporary or downloaded media. Removing downloads or reinstalling a streaming or social app may free space, although that space might be counted under the app rather than System Data.

Check what the app keeps locally before deleting it. Reinstalling can remove downloads or other data that isn't synced.

A full erase and restore is a last resort, not the first thing I'd try. It can be a hassle and doesn't work for everyone. If you do consider it, make a backup and check that anything you need is safely stored before erasing.