Press and hold the Side button and either Volume button until the power-off slider appears
Drag the slider to the right to turn off the iPhone
Wait for the screen to go completely black
If the screen is frozen, quickly press and release Volume Up, quickly press and release Volume Down, then press and hold the Side button until the Apple logo appears, and then shut it down normally after it restarts
You can also go to Settings > General > Shut Down, then drag the power-off slider