So there is this game mode called MyTeam. Everyone playing MyTeam have to build their own best team, one way is to open packs so you get players, the players have different tiers so the better the rarer etc. So the other way is to directly buy players what you want from the market, where everyone can sell and buy those players from packs, and again, the rarer the player the more expensive it becomes. Hope you got the idea. There are also ways to get the players like from quests or tournaments etc.