Press and hold the Side button and either Volume button until the power-off slider appears
Use AssistiveTouch if enabled: go to Device, then Lock Screen, then press and hold the Side button to power off
Use Siri to open Settings, then go to General, then Shut Down
If the screen is unresponsive, force restart the iPhone using the button combination for your model
If the buttons do not work, let the battery drain completely to power it off