It started with some locked crates. Inside those, projectors and cryptic slides. Those slides led to– actually, this goes on for quite a while. It’s complicated, there are a bunch of steps, and it took a lot of hard work from the Call of Duty community. Instead of my half-assed explanation, watch this summary from Zombies YouTuber NoahJ456:
Source: Destructoid The Call of Duty community finally solved Activision’s convoluted Black Ops: Cold War ARG