Its 30 vs 30 so how does it work? Do they get randomly drawn into teams live, right before they play?
Hoping this link helps explain things a bit!
https://smash.gg/tournament/twitch-rivals-league-of-legends-role-robin-showdown/details
But basically you play with/against the other 29 players in your bracket to get the most wins possible. The player with the most wins from each role forms a finals team to play vs the other bracket’s finalists.