How do I restart my Android phone when the screen is frozen?

That combo is Samsung and most others. On a Pixel it's different, just hold the power button on its own for about 30 seconds, ignore everything, it restarts. If nothing at all responds, let the battery run flat, then charge it for a while before turning it on.

hold the side (power) button AND volume down together, and keep holding for around 7-10 seconds. Ignore whatever pops up on screen, it'll go black and reboot. That works even when the side button alone is set to open Bixby.