Press and hold the Side button and either Volume button until the power-off slider appears, then drag it
If the screen is unresponsive, press and quickly release Volume Up
Press and quickly release Volume Down
Press and hold the Side button until the iPhone turns off
If needed, let the battery drain completely
If the iPhone is frozen, force restart it instead:
Press and quickly release Volume Up
Press and quickly release Volume Down
Press and hold the Side button until the Apple logo appears, then release it