That being said, this kind of stuff is gonna mostly be in the papers, so I suggest following the interesting references from the article, and repeating until you feel you understand :)
Rather like the 1957 development of cross-validation (which I remember reading, but definitely don't have a cite for), the double descent thing is part of the paper that is used on the way to something else.
It looks though that they have a double descent on the train set too, so it might not be the same phenomenon.
Nevertheless, good to know, thanks for sharing! I knew both papers but never thought giving much attention to such details of the figures of the 1998 one. Is the connection between the papers well known, i.e. something people talk about?
Yeah, I do mean those graphs, it seems to be the same phenomenon at least.