How does Apple the trade-in work?

With Apple Trade In, you can get a great value for your current device and apply it toward a new one or a gift card. Depending on the condition and model of your iPhone, you will receive a value for your device, or if your device has no Trade In value, Apple will recycle it for free.

⇢ Apple Trade In - Apple

-Zachyy ✌️

Good point IdrisSeabright, and if you decide to sell your iPhone yourself, make sure to remove your personal information and have your device ready to sell.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

You can often get more money by selling the phone yourself. However, you have to deal with the hassle of selling it yourself.