Fully turn off any iPhone or Android phone — the buttons changed, so here's the current way.
Press and hold the Side button and Volume Down together until "slide to power off" appears, then drag it. (Holding the side button alone just triggers Siri on these models.)
Press and hold the top or side button by itself until the slider appears, then drag to power off.
Hold the Power button (on many newer phones, Power + Volume Up) until the menu appears, then tap "Power off." Some phones have a Power option in the Quick Settings shade.
Wait for the screen to go completely black and stop responding. To turn it back on, hold the Power/Side button until the logo appears.