When you say "hey google" currently the whole screen gets
covered with that program in higher priority than the program
you are in (say Chrome or the Android Dialer app).
But with the new markup and hardware support only a small
LED like section be lit up brightly in the corner showing that
your request was received. Clicking on it will bring up the
Google Assistant
Also when you darken the screen, or it is automatically
darkenned because of a low battery or sunlight, there is still a
region that can potentially override the darkened state when a
flashing led has to be shown (incoming call, message, etc).
When manually lowered, the screen stays dark.