Mushrooms contain a fruiting body (the part of the mushroom you'd recognize) and mycelium, the part that grows underground or inside the plant. Think of the mycelium as the roots of the mushroom. Nearly all mushroom products are mycelium, not the actual mushroom. They're grown in a lab on sterilized grains (such as rice, sorghum or oats) and ground into a powder.
If you're buying a mycelium product, you're not getting the beta-glucan-rich content of the fruiting body. Beta-glucans are the active compounds in mushrooms. Fruiting bodies are full spectrum and also offer more vitamins and minerals. You're not getting a real mushroom and your dog is not getting all of the health benefits.