Restore to factory settings

If you want to know how to reset to Factory Settings read this: Use iTunes to restore your iOS device to factory settings - Apple Support

If that's not your question then what is the question?

Are you asking how? What is your question?

To do this on the device itself:

Settings/General/Reset/Erase all Content and Settings

You'll need to disable Find my iPhone/iPad to do this.

The procedure will return the device to an out-of-box configuration when it has completed.

Restore to Factory Settings via iTunes reloads the iOS software. "Erase All Content and Settings" doesn't do that.