iphone set at factory setting
what is the itune website to go go
iPhone 7 Plus
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This article talks you through using iTunes to restore your device to factory settings.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
This allows you to do same from the device.
How to erase your iPhone, iPad, or iPod touch - Apple Support
Only the first will replace the iOS (ie the operating system) at the same time as removing content and resetting to defaults.