To people who says “they aren’t airtight” are not correct. Living in a country where building houses like this is the norm I can say that they are build to be as airtight as possible to keep in humidity and thereby heat inside. What makes them livable and not a humid moldy plastic bag is a well tuned mechanical ventilation system with a heat regeneration system (air cross flow system, really simple actually) that recovers most of the heat. The ventilation system runs 24/7 and keep the air fresh, more fresh than most conventional houses in fact if it’s tuned correctly that is. But yes the house is as airtight as posible and they pressure test them to ensure they meet the standards. The biggest issue with it is actually cooling them. We have issues here in summer because no method of removing heat is properly implemented yet. But in winter it’s awesome. My heating bill is practically 0.

Genuine question: if they’re airtight, how do oxygen and CO2 levels remain livable? Spacecraft and submersibles require oxygen supply and CO2 scrubbers to keep occupants alive.

how do oxygen and CO2 levels remain livable?

The ventilation system was explained.

I suppose what trips me up is that my brain sees the concepts of “airtight” and “ventilated” to be in conflict and I am seeking further clarification than what has already been provided.

The idea is that air exchange mostly happens through a heat exchanger, so the air leaving the house warms up the air that is entering the house, for this to actually work all other parts of the place need to be airtight, because otherwise your heat escapes.