Not all mushrooms produce spores all the time. A mushroom is similar to a piece of fruit on a tree, the seeds symbolic of the spores. Mushrooms, like fruit trees, only produce fruit and seeds for reproduction if there are enough available nutrients and water to account for the extra energy required to produce the spores. Therefore, most spores are characteristic of a fungus system that is healthy, because it is reproducing.
Spores either drop, or are ejected from the bottom of the mushroom cap. The miniscule size of spores allows them to get caught in, and carried along, gentle air currents. When most spores hit the ground, they fall on infertile ground: rocks, leaves of grass, streams, etc. The few spores that do fall on fertile ground send out shoots into the ground, finding other shoots, from other spores and starting a reproductive process, connecting and expanding the underground fungus system.