Forgive the poker newbie-ness What is the difefrence between WPT Barcelona and the WPT Main Event. Is Barcelona a feeder to the main event, or totally seperate?

Also, when you join a feeder online, say you join PokerStars who have their own network and enter the WPT feeder, are you up against players on the PokerStars network only, or are you up against players from all poker rooms? What I am getting at is, does it make sense to join a less busy room on its own network to play a WPT qualifier than a big room like Pokerstars?