How do you initialize the factory when you sell used iPhones?

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When you sell used iPhones, you tell me to initialize the factory. How do I do this?

And another thing I'm curious about is that if you initialize the iPhone factory, is it safe to sell it as a used phone?

Computer  In the case of hard disk, it is said that it is better not to turn it over because you can recover files even when you sell it secondhand.

  116 2   7 years ago