One of the reason the last model is a new scale of complexity might be that it somehow use "old school" techniques: There is now collapse, no circle packing, it's just straight traditional (hard!) folding from start to finish. Creating such complex models from the traditional tools is usually really hard. It also has a certain elegance to it that is often very pleasing.
As I understood it, after (1) planning out your design you then (2) create a flat set of pockets with as many pockets as you need appendages, of appropriate sizes, before (3) extruding those pockets out into limbs, antennae, wings, and eyes.
Having it explained visually, it makes so much sense. A bit like a chef doing mise en place before then constructing a dish. Absolutely fascinating.
For something that still touches on some of this stuff but is more beginner/child friendly, I'd recommend Jun Maekawa's book Genuine Origami (https://www.amazon.com/dp/4889962514)
There's also the OSME books (http://osme.info/) which collect origami related papers.
Robert J. Lang has been an avid student of origami for over fifty years and is now recognized as one of the world’s leading masters of the art, with over 700 designs catalogued and diagrammed.
Robert Lang and Erik Demaine were also featured, IIRC.