System data is category of data that takes up space on your iPhone/iPad. They include temporary files, log files and media buffers. These are created as you use your phone.

They should be purged after they are not needed anymore but they aren’t being reliably purged on iOS for whatever reason.

Anywhere from 3 - 4 GBs consumed is a reasonable amount of storage use. But these files tend to take up over 25 GBs and in extreme cases, over 100 GBs.

OP posted a reliable way of clearing the unwanted data manually.