The regulation calls out 19 vehicle functions which must have physical controls inside the vehicle, with additional requirements about size and usability of the buttons.
Those functions include:
Turn signals
Hazard lights
Horn
PRND gear shifter
Driver assist function, if present
Windshield wipers
Windshield defroster
Power windows
Emergency call system
Power off switch for EVs
My BYD Atto 2 only has a physical power button, no touchscreen power off. (and an app, but that’s another matter)
What happens if you power it down, do you need to press the button to get the car to activate beyond opening doors / 12v stuff?
Yes. A few automatic things like lights staying on until you’ve locked the car, windows auto closing, but otherwise pretty much what you’d expect from turning a key. Doors are manual, so that’s not affected.