I think you're missing something. DCS World is the "main game". All of the various aircraft modules (P-51, Mig-21, F-86, UH-1, Mi-8) effectively "plug into" DCS World. You can't use any of those modules without DCS World.
As for the maps, the Caucasus is the only map that is currently available for DCS World. As cichlidfan pointed out, we have the Nevada and Straits of Hormuz coming some, but it's just the Caucasus for now.