First, APFS-- we talked for Android on Ext4.
Chances are good, as long as you're looking at iOS 10.3
and later, that you are going to have APFS.
The data stored in APFS is primarily
going to be databases and plist files.
It's enabling full-disk encryption,
which has been in play for a very long time on iOS devices.
Remember, garbage collection is not
going to go through and clean up everything,
but you will have database vacuuming in place.