Windows 10 is the "last Windows ever". There's a pretty wide tree of branches under "winmain" that teams contribute to, and at each layer more branches are merged together, until they finally hit winmain, and then weekly they tag (more or less) a build from winmain as the Insider's build for the week.
It's actually a lot more elegant than you'd expect it to be for a project with as many developers as Windows has
