The data lost after the factory reset is mainly your personal information. These include apps, messages, contacts, videos, photos, music, calendar, etc. However, you can recover all of these data from your iTunes or iCloud backup. Turn on a new or recently erased iPhone.
Tap Set up manually, tap Restore from an iCloud backup, and follow the on-screen instructions. No, not even IT professionals will be able to recover their data from the phone's data storage. Rather it's during an update, or if your iPhone has a problem, your iPhone's data is no longer there.