AFAIK all mycelium helps root systems, from the symbiotic relation, but in indoor, hydro type grows it might be harder.

What would be interesting is the combination of vertical mushroom grows and vertical hydro culture.

Imagine a tube/tower of mycelium, but also allowing the cannabis to grw out of the tube/tower... and having the root systems intertwined, but the problem therein is that the diff in the mycelium being perennial as opposed to the cannabis being annual...

I've experimented with vertical farming greens and mushrooms together; figuring out the schedule for the different colors of light can be a bit of a pain, but I used LEDs. - I stacked tote bins that I had drimmeled to interlock with eachother like a greenhouse. I don't think hydro would be possible without a super specialized fungus. If you ward off infections, regular hardwood based edible mushrooms work in dirt with vegetables. The mycelium doesn't seem to particularly help though. Greens and most edible mushrooms come from different substrates. I've yet to really look, but I suspect most desirable mushrooms won't be the best mutualists. There will probably be some that are more than a little valuable in balancing a polyculture though. I really want to find those pairs and groups, like we've discovered in say oaks or orchids, where they seemed to have evolved together.

Aside: Mushrooms are perennial but the mycelium doesn't always die away. And some are selfish in not so mutualistic ways too.

the co2 from the mycelium, let it build up, then periodically cycle into your vertical farm to increase the co2 concentration -- then watch your vegetables or whatever grow bigger!

Nitpick, but there are certainly parasitic fungi which do not have a symbiotic relationship with plants. Also only some fungi have a mycorrhizal relationship with plants. Most are saprophytic meaning they decompose dead organic material.

This is of course extremely beneficial and required for a closed ecosystem, but the symbiotic relationship isn't as direct as mycorrhizal fungi and is really part of a much more complex network of soil organisms that all need to be at play. Simply introducing a saprophytic fungi to your soil will probably not result in any benefits to your indoor cannabis garden.

My understanding is that Mycelium mostly helps with nutrient motility, moving them across roots of the plants. Potentially increasing bioavailability.

Some Eukaryotes are also reported to my mycellial connection with the plant (i.e. merge with the plants) some sort of forest underweb sort of thing, but I think that would be very difficult to reproduce insitu. Eukaryotes are amazing but also so diverse they defy classification.