On the iPhone X, XS/XR, iPhone 11, and iPhone 12, Apple assigned new functions to the Side button on the side of the device. It can be used to activate Siri, bring up the Emergency SOS feature, or other tasks. Because of this change, the restart process differs from the method used on earlier models.
To restart (soft reset) the iPhone 12 or iPhone 11/XS/XR/X, follow these steps:
Press and hold the Side button and Volume Down buttons at the same time. Volume up works, too, but using it can accidentally take a screenshot.