In order to participate in regional competitions, one must acquire 8 Gym Badges signifying the defeat of that region's Gym Leaders.
The Elite Four are four Trainers who are regarded as the toughest in their regional Pokémon League, short of the Pokémon League Champion. Those who challenge the Elite Four must have won eight Badges from that region and face all four and the current Champion consecutively without losing to any of them. The rules remain the same throughout all Leagues, however, Champions are allowed to make their own rules as long as they are fair to the challenger.
One group resides on Indigo Plateau in Kanto and Johto, another is in Ever Grande City in Hoenn; three more reside at the Pokémon Leagues in Sinnoh, Unova and Kalos.
In Galar, their League is compromised of something called The Champion Cup. In this, instead of the traditional Elite Four, a Trainer will participate alongside fellow Trainers who have also won eight Gym Badges. They will all face off against one another, proceeding to the next battle with every win with the overall winner then facing a final three Trainers, finally facing the Champion if all three battles are won.
There is not a limit to how many challengers can battle in the tournament, and competitors battle line-up are chosen at random depending on their "scorecard" of their performance in their Gym battles. The final three Trainers are Gym Leaders that battle in order to defeat contestants and then for themselves be able to face the Champion, in the hopes of becoming Champion.