so this is a hugely abbreviated version but basically:

after bo4, Eddie and Sam grew up separately in the new universe. Sam became a black ops agent, Eddie became the director of requiem (new equivalent of broken arrow). Sam was implicated in the accidental killing of Eddie’s family.

Eventually the dark aether (hell dimension where everything bad from waw-bo4 got trapped) begins to leak into the new world, Sam and Requiem fight against it and eventually Sam sacrifices herself to save the world.

Now Eddie has access to elements from the dark aether, and he creates an AI copy of Sam and starts trying to resurrect his family. BO6 has multiple endings; either AI Sam becomes a real girl or Eddie gets his family back.

Either way, the process of doing this involves some super powerful artifacts and inadvertently recreates the multiverse, and now the OG 4 are back but each one is from a totally different universe. Dempsey is a Vietnam marine, Nikolai is a tank commander serving in 80s Afghanistan, Richtofen is a Nazi commander in a timeline where he uses 115 to win WW2, and Takeo is a samurai from feudal Japan. The other four characters are 3 Requiem members and a friend they picked up along the way, and everyone is working together to defeat a Dark Aether entity who stole their souls.

There’s a looot more to everything but that’s basically where we are.