Switching between electricity and petrol – environmental impact

Question from a technically completely untalented V90 driver: when driving in “hybrid” mode and the V90 switches between power and petrol drive again and again (depending on the road situation and speed), what about the burner: does the “empty run” continue when it is streamed or is it automatically switched off? If the latter is the case: wouldn’t that be nonsense with regard to the environmental impact, since a high CO2 load arises, especially when starting the engine? Thanks for expert feedback.