Is there any criteria? How are the players paired? If you know pls tell me below
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It depends. There isn't one answer to this, because it differs for all variant world championships held - the organizers are all different players.
You can find more information about every variant WC in its respective team:
lichess.org/team/antichess-wclichess.org/team/atomic-wclichess.org/team/crazyhouse-world-championshiplichess.org/team/3-world-championshiplichess.org/team/horde-world-championshiplichess.org/team/racing-kings-wc