Agaricus bisporus, the white button mushroom, is a fungus. It has no roots or leaves. Its body is a web of fine white threads called mycelium, and what you sell is its fruit body.
In nature it lives on rotted horse manure and straw and cannot digest fresh straw. So the grower first ferments straw and manure into compost the fungus can eat and its competitors cannot. Then the grower adds a moist soil layer, the casing, because casing supports reliable commercial fruiting. Everything else on the farm serves those two facts.