Make sure you have the right model, as some Xbox One controllers won’t pair.

The iPhone and iPad will finally feel like proper traditional game systems this fall, as iOS 13 and iPadOS 13 introduce support for some Xbox One controllers and PlayStation DualShock 4 controllers. We’re focusing on the Xbox One S controller in this article, so you’ll want to head here if you need to hook up a DualShock 4. You can try this feature now if you want, but keep in mind you’ll need to download the still-buggy iOS 13 public beta.
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https://www.macworld.com/article/3410800/how-to-pair-an-xbox-one-controller-with-iphone-or-ipad.html