Nuketown 2025 and Nuketown zombies are two separate maps that had two separate codes. If you bought the hardened/care package edition you got both codes. If you pre ordered the regular edition, you got Nuketown 2025. If you bought the season pass you got Nuketown zombies.
Nuketown 2025 is not in regular map rotation and can only be found in Chaos Moshpit under the "Bonus" section.
Nuketown zombies can be found when you enter the zombie menu.
That is as simple as I can make everything lol