All competitors in the Galarian Star Tournament (except for Piers, Sordward, and Shielbert) will Dynamax or Gigantamax the Pokémon marked below at the first opportunity if their partner has not yet Dynamaxed or Gigantamaxed themselves. Several Trainers require a certain number of tournaments to have been won to unlock them as partners.
In The Gathering of Stars, Leon invites several trainers from the Galar region to the tournament. At the end of the episode, when everyone gets together, he starts the competition.