Where do mushrooms get their energy from?
Mushrooms are the fruiting bodies of fungi. Fungi get their energy by decomposing dead or decaying organic material, such as fallen leaves or dung.

Some background: spores of fungi germinate forming mycelia consisting of threadlike hyphae. When the hyphae of different fungi meet, they may have sex and form mushrooms. The mushrooms produce the spores. See https://en.wikipedia.org/wiki/Fungus.

So, each of these stages get their energy ultimately from decomposition, although the mushrooms get their energy from the mycelium. Spores rely on their internal energy storage to germinate and start decomposing. Even though spores are tiny, the have energy stored, like all cells do.