> all the ingredients necessary exist in the soil the mushroom is grown in
I can't speak for mushrooms, but for plants, the vast majority of the mass actually comes from carbon in the air (breathes in CO2, takes the carbon, breathes out O2) and obviously water.
>Generally, plants make their food using the sun's energy (photosynthesis), while animals eat, then internally digest, their food. Fungi do neither: their mycelium grows into or around the food source, secretes enzymes that digest the food externally, and the mycelium then absorbs the digested nutrients.
I can't speak for mushrooms, but for plants, the vast majority of the mass actually comes from carbon in the air (breathes in CO2, takes the carbon, breathes out O2) and obviously water.