This is the most popular official "offline" COD game for mobile. It features both a campaign mode and a survival mode.

They burst out into the cold morning air, sliding down the snowy embankment as the facility vanished in a cloud of fire and ice. They lay in the snow, chests heaving, watching the smoke rise against the dawn. The virus was gone. The threat was neutralized.

To understand why this is such a big deal, you have to look under the hood. Most simple games come as a single (Android Package Kit) file. You install it, and you play. But a game like Call of Duty is a beast. It involves massive textures, complex 3D models, and high-fidelity audio.