power button only brings up the assistant, how do I switch my phone off

which phone? on a samsung, hold the side button and volume down together and you get the power menu. to change it for good: Settings > Advanced features > Side button, and set press and hold to Power off menu. on a pixel it's power + volume up, and the setting is under System > Gestures > Press and hold power button, turn the assistant toggle off. the power icon in the quick settings pulldown works on both if you just need it off right now.