A mycelium may fill a single ant, or cover many acres. If the mycelium produces microscopic fruiting bodies, people may never notice the fungus. Most fungi build their cell walls out of chitin. This is the same material as the hard outer shells of insects and other arthropods. Plants do not make chitin.
Fungi feed by absorbing nutrients from the organic material in which they live.
Fungi do not have stomachs. They must digest their food before it can pass through the cell wall into the hyphae. Hyphae secrete acids and enzymes that break the surrounding organic material down into simple molecules they can easily absorb. Fungi have evolved to use a lot of different items for food. Some are decomposers living on dead organic material like leaves.