What would be the best ways to find the MOST active video game communities for specific games? I could make a couple guesses, but how would I find which ones for sure? Never involved myself in any of those before...

I’m not entirely sure what you’re asking.

Most games/communities in here have discords which is where all of the action happens so to speak. Discords are usually where everyone gathers to talk, route, ask for help etc.

Discords are usually linked in the forums for the games, so check there to see if the games you’re looking at have a server setup.

https://www.speedrun.com/games you go there, and it automatically sorts by active players.

Awesome. I think that will work vae. Then I can look more in depth for a discord as timmiluvs says.