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 oxygen level inside is lower than outside. It might affect your health.
I can’t tell if you’re joking or not. Unless a home is small, effectively airtight, filled with people, and the doors and windows are never opened, oxygen concentrations aren’t going to fall enough to be impactful.