I used cheap potting soil from the store.

lol: now you know where do they get the stuff to sell you :)))
(well, just kidding).

in fact, it is an interesting question where do mushrooms come from, to which there is no universal answer. yes, spores can be carried by the wind, flies and other insects that crawled over the mushroom. and the mycelium can develop and doze in the soil for years (!) until suddenly something changes and the environment shifts in the direction that the mycelium considers favorable for reproduction. and then it grows a fruit body, and we say: oops, a mushroom has grown! .. something like that. everything is nonlinear. - !ENGAGE 50