Well, yes, Mushrooms are sort of related to plants, but not.
Fungi are not animals, for sure. Like plants, they do have a root system and also bloom, the mushroom itself is the bloom, for reproductive purposes.
But mushrooms lack the green chlorophyll that’s absolutely essential for true plants to create food for themselves. An argument could be made (and it has) that mushrooms behave more like insects than plants.
It’s the function of fungi to break down plant matter in the soil, much as ants, worms, and larva do.
Not only is fungi found on trees, leaves, and other dead material on the forest floor, but it can also be found along walls and in foods, anything that’s capable of rotting.
The mushroom simply couldn’t be boxed in to any one category, so the kingdom of Fungi had to be created to describe these organisms.
A fungus is classified as a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts, molds, as well as our humble mushroom.