I wonder if this has a negative effect on the amount of fresh air that they get
Molds are a problem and there is another, for rodents, mosquitos and cockroaches this is a dream made true. Spiders can take mosquitos easily, but the other two will need a plan
Plants can be net CO2 consumers, but they burn O2 to CO2 just like you. Tropical forests are net CO2 producers for most of the day, except for a few hours around noon, but that's enough to become net consumers averaged over the day. Occasionally someone fills their room with plants, and then puzzles over having high CO2 levels.
Mine would love that environment I’m sure though.
My mother has a decent sized greenhouse. Her cats spend most of the winter in there by choice - they love it.
The greenhouse effect makes the air inside a lot warmer than outside. This reduces its density, and it becomes buoyant and spills out the roof vent.
There may also be fans, but you can get some ventilation for free (energy-wise).
Dont know if they have one set up, but they could. But possibly the sun already warms it enough even in winter that it is not needed even with natural ventilation.
Heat exchanger: https://en.wikipedia.org/wiki/Heat_exchanger#HVAC_air_coils
To be clear I did not mean HVAC, which requires (some) energy in itself.
At home I do not have HVAC. However, the outbound air passes through a rotating metal structure that sucks up the heat from outbound air and releases it into the colder inbound air. Just using the temperature gradient that is there naturally. Passive energy transfer.
Same principle can be used with hot water (e.g. from showers).
Then one can put HVAC on top of this for active warming.
At least in Finland, newish office houses typically don't even have windows that can be opened; this seems to be a surprise to any foreign visitors.
That's because the costs for heating of the building during winter are way cheaper than a better thermal isolation of windows and a good ventilation system combined.
I live 55°N in Russia so I know that from practice, but we don't lock the windows throughout the summer because the cooling would cost around the same as heating in winter, just one or two opened windows in the night for convection and curtains in the daylight are enough to keep the temperature.
These sentences contradict with each other. The cost is not only payments for service, it's a sum of resources, time and work needed to make and maintain. You can't make a thermal system cool itself passively due to the 2nd law of thermodynamics.
The difference between Helsinki and the place I live is that they don't have temperatures above 30°C while here I have those at least 2 weeks straight each year. Passive ventilation stops in those temperatures because there are no wind outdoors besides the hot columns of air from heated surfaces, HVAC costs are the same as winter's heating while it's PITA to clean them from ice each spring, a window ventilation during the night refreshes the air good enough.
So the "passive" houses are not at all passive in terms of technology (to the contrary, they are quite dependent on technical solutions enforcing airflows and exchanging heat up or down, to the point of being vulnerable; the point is just that this is not done through windows because humans intuitively don't do it so well or optimize for the whole building).
Some years ago, there was an electricity outage in the area where my office was. The basement floor had our server room with plenty of blade and rackmount servers, and some 30 kW of UPS capacity - which had been unused for years because electricity outages are extremely rare here. UPS is all very well to keep servers running, but this happened in the summer and the server room cooling system was not behind the UPS like the servers were. Thus, we suddenly have a 30 kW unbalanced heat load to the basement floor... Result was some frantic running up and down. None of the windows could be opened. We could open garage doors in the basement floor, open all the doors to stairways and halls, and run the the top floor which has the usual sauna compartment with doors to a balcony - opening all these, we could create a chimney effect to get airflow through the premises and prevent our servers from melting on the spot while we were shutting them down gracefully in priority order.