Press and hold the side button and either volume button until the power-off slider appears
Drag the slider to the right
Wait for the iPhone to turn off completely
If the screen is frozen, press and quickly release volume up, press and quickly release volume down, then press and hold the side button until the Apple logo appears, and try again
On iPhone models with a Home button, press and hold the side button or top button until the power-off slider appears, then drag the slider to the right