Usually both, in my experience.

You’ll have fans for plants that firm up stems and stuff, in addition to helping with transpiration.

Then you’ll have fans or passive vents near the highest point that move air outside, usually for the sake of venting excess heat and humidity. If you live somewhere cold, these probably stay closed through colder times and are active/open only during the height of summer (these are often the passive vents). If you live somewhere warm, it’ll probably always be active fan-driven, and usually running.

A lot of people have their ventilation system hooked to climate control sensors, so it only engages when it crosses a certain threshold.