All one has to do to unlock Call of Duty Mobile's Zombies mode is reach level 5 through playing the rest of the game normally. Once that's done, players are free to play the mode as much as they want while it's available.
Exo Zombies is a downloadable game mode in Call of Duty: Advanced Warfare. This mode is available in each DLC pack. Exo Zombies is very similar to the Zombies game mode in Treyarch's Call of Duty games. 1-4 players (online) or 1-2 players (local play) must survive countless rounds against zombie hordes.