Hopefully new player questions.

I've been trying to find what COD is free. I downloaded "Call of Duty" on Steam, and now there are other tabs inside that game (launcher, really), and Warzone says download, but then it says BO6 is purchasable. I see options to download the files. Then mw2 says install, but it's asking me to install DMZ. I am confused i just want to know which CODs are free to play and how to download them...