My iPhone is stuck in a demo loop

You have a stolen device. Demo units are locked down. They are only for display in retail environments and are not for sale.

Return it to wherever you got it and get your money back or turn it over to the police.

Apple can track the location of it, by the way. It’s best you do the right thing before the police show up at your door…

If it's boot looping, try a force restart. Press and very quickly release the volume up button, press and very quickly release the volume down button, then press and hold the side button for about twenty seconds. When the Apple logo appears, release the button. The timing is a little tricky, so try it a few times.

Force restart iPhone - Apple Support