I know that a greenhouse is not air tight so I imagine it breathes somewhat and exchanges gas with the outside. However, I also assume that it’s going to use up the co2 in it for plant growth. How does it not run out? I think you could vent it, but then wouldn’t that cool it off and thus defeat the purpose of a greenhouse? All my web searches either give climate change facts or co2 supplementation by farmers.
The same reason your house isn’t a pure “air” environment.
Carbon Dioxide is heavier than all the components that make up the air you breathe. In a greenhouse they have fans that move air around, because stagnant air would generally be bad for both plants and humans. You’ll notice a gap in all greenhouse environments to the ground.
CO2 gets pulled in, but eventually pushed out because of its weight, but enters in from the ambient atmosphere being injected by the fans.