I've got a CS related degree, but I'm a mostly self-taught dev, and not understanding 90% of the code really makes me reevaluate my career choices. Where and how can I learn this stuff?
If it's the former, go for it.
If it's the latter, don't worry about it. I have a CS degree, did 6 years on a team writing C++ at Microsoft, and only briefly flirted with understanding this stuff. A couple of the wizard devs on the (very large) team could do it, but they also knew that code needed to be simple above all else, so it was effectively banned in our code base.
The other is building up basic "data types" by pretty standard lambda calculus > LISP route. "Understanding Computation" book has a great chapter 6 on that, which is available in blog & video forms on https://computationbook.com/extras - Here, Church numerals were used to represent numbers. - booleans & conditionals here didn't resort to the lambda representation you'll see in the book, but relied on type conditionals builtin to TypeScript. - The names "Cons" & "nil" are a ringer for LISP-like building of lists, and recursive processing of lists, from a "pair" data type.
What's common to both approaches to building arithmetic is starting from zero + a "successor" function T. That approach is called "Peano arithmetic".
I still recommend that post/video (and the book in general) but I have to admit there is no 1:1 correspondence to the TypeScript going on here.
Still, it'll teach you some general maneuvers for bootstrapping computation out of almost nothing , qnd once you're comfortable with those, you can read things like this TypeScript post, or aphyr's original Haskell post, which bootstrap computation out of sjighly different" almost nothings" and without following the details still have a high-level idea of where it's going (like the poor interviewer in the story ;-)