A lot of mushroom farming takes place in the lab. Inoculating sterile substrate and sterile grain with mushroom mycelium. Working with liquid culture and grain spawn to expand the mycelium for large-scale mushroom production.
These gourmet mushrooms are wood-loving species, meaning they don't grow in dirt or soil. Normally these mushrooms would be found in nature growing off of trees. To simulate that environment, sawdust is used as a nutritious substrate for mushroom cultivation.