That you get through the launcher, of course it’s not hosted on GitHub, you can download several different versions of Minecraft from it, there’s no trouble with that.
I never wondered how they get them exactly, but I think it might be a direct download from the official sources and then some patches applied after the fact to “crack” it (do note I could be totally off on this one).
You also get mod support with it, I instantly got the gamepad mod for example and it’s all been working flawlessly, though the documentation could use some work, it was quite confusing to me.