With the newer model iPhone's including the iPhone X and iPhone 11 models, Apple changed the way you power off / turn off your iPhone.
To turn off your iPhone you need to press and hold the button on the right side of you iPhone and at the same time press and hold either of the volume buttons.
Doing this will bring up a display that will allow you to turn off your iPhone. It will also give you the option for you to view the Medical ID if it is setup on the phone and also trigger the emergency SOS feature.
To turn your iPhone back on just press and hold the button on the right side of the iPhone.
Hope you found this useful!