I haven't bought a game of this series. I was wondering if they have coop and if yes, how it is implemented.

The game sadly don't support PvE coop natively. But you can play Cooperative vs matches involving multiple players on ether team from the in-game multiplayer modes. The game modes are Assault that involves one team is attacker and the other defender, and Flagcontrol where the 2 teams strive to control the fag points to win.