If you want to learn more about this stuff, the guy in the video wrote a great book called Origami Design Secrets (https://www.amazon.com/dp/1568814364/). It's certainly still the book about origami design.
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.
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.