iPhone power button

Turn off iPhone

• iPhone with Face ID: Simultaneously press and hold the side button and either volume button until the sliders appear, then drag the Power Off slider.

Turn iPhone on or off - Apple Support

The Always-On display setting is on by default. To turn it off, go to Settings  > Display & Brightness, then turn off Always On Display.

Ahhhhh. Try a forced restart. First take off any case. Then:

Press and very quickly release the volume up button. > Immediately press and very quickly release by the volume down button > Immediately press and hold down the side button and keep holding the side button until the Apple logo appears, release the side button.