They aren’t “airtight”, that would awful. They are well-insulated and designed to take advantage of passive solar heating and air exchange cooling. The way roofs and windows and orientation on the land is usually done for western homes is just terribly inefficient for capturing and releasing heat in the right ways. Just some thick walls, a bank of windows facing the sunrise ,and some proper roof vents that can be opened when it’s hot is all most passive houses really are.