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.