Plants make their own food from sunlight. Fungi cannot. A fungus feeds by pushing thin threads into dead plant material and digesting it from the outside. Wheat straw, paddy straw, sawdust, cow dung: these are its food.
This is why mushroom farming needs no field and no sunlight. It needs a clean growing medium, called the substrate, and a room with the right warmth and moisture. The mushroom on the plate is only a small part of the organism. Most of it stays hidden inside the bag.