You'll want to do this before selling your iPhone or trading it in. Restarting it will just give it a refresh while keeping all your data. Why does it differ based on iPhone model? As Apple has ...

You can back up your iPhone and restore all your data through iTunes, iCloud, Finder, or Google One. Here’s how to get it done.

This is the "nuclear option" - a factory reset will wipe all of the apps and data from your iPhone, and return it to the state it was in when you first took it out of the box after purchase.