I threw a spent oyster mushroom grow block into my Jalapeño plants

Not usually. They often exchange nutrients with one another. This is generally as there are fungi that can eat plants and whatnot.

They are very symbiotic! I'm no expert, but there's several ways they are symbiotic.

There's the cycle of plant dies, mycelium eats/decomposes it which spreads the nutrients through the soil, new plants have more available nutrients.

There are also lots of cases of them living symbiotically together. Basically the fungus and plant exchange nutrients with each other.

https://www.britmycolsoc.org.uk/mycokids/some-fungi-help-our-planet