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