Couple of really good explainers and, just in general, channels worth perusing for DRG info.
This is long, but time stamped and full of great beginner info.
And this guy does some quite entertaining stuff on it too.
This is a good way to get a feel for the development path over the years.
This is ostensibly an endgame (promotions, deep dives) explainer, but actually has a lot of other good info in it.