It’d be nice but I don’t know what you’re asking for would look like exactly.

Academic papers usually have an introduction just past the abstract and may be the closest to what’s requested here. This is supposed to get the reader prepared for the meat of the paper. In a lot of math papers, it acts as a 101 prepper/refresher for the core concepts used in the paper.

There’s also the wolfram site for math. It’s been a while, but it functions something like an encyclopedia of all math topics. I’ve used it to quickly backfill a reference many, many times.

Reading an article or proof is not exactly analogous to “reading the source” as both articles and proofs are meant to be written to be read by a human with an agreed upon background and lead them logically to the conclusion(s) of the article. Articles and even proofs can be intimidating, I get that, but they are surmountable with a suitable time investment.

I think maybe Wikipedia is better here than Wolfram's site, it has entries for Ramanujan graphs and expander graphs for example.

The paper does actually seem readable, especially at the beginning, but it's dense and long. Somewhere between the article and the paper in terms of complexity and technicality would definitely be interesting to me, although I realise it's a lot of work to produce something like that.

Interestingly the paper acknowledges collaborators who have helped via MathOverflow. I wonder if a collaborative site for explaining maths papers in more detail could be viable. After all, even xkcd.com has explainxkcd.com. Like you, I am not sure what this would like, but it would be fun to find out.

It kind of sounds like they think they found a kernel of knowledge which could form the body of something like a for-loop or inductive step of an induction proof of the larger problem.