According to a scientist by the name of Sir Isaac Newton, “all matter seeks equilibrium”. Cold air and warm air seek each other out and mix together. This is the same as if you were to combine two glasses of water, each with a different temperature. The water would “mix” together forming a consistent temperature.
All buildings lose heat to the outside or gain heat from the outside. That is the reason why structures need to make up for heat loss and gain. And we do that with heating and cooling systems.