Having a very "to make an apple pie from scratch first you must invent the universe" sort of day. Consider contemporary popular music; its beats are still recognizably, unmistakably influenced by synthesized drumbeats from old analog synths. E.g. you could make an entire movie about just the Roland TR-808. So I look up 808 schematics and see a couple of opamps at the heart of the bass drum signal-generation module. And after a while of puzzling over it the other day, I think I feel in my guts that the behavior of an ideal opamp is pretty simple and nice, so, come to think of it, how is your garden variety 741 implemented in transistors, anyway? Oof, that looks like a giant mess. Let me just try to understand a small piece of that. Like, what's a current mirror? How does it work? I get one modelled and working in a simulator, but still kind of don't understand how it works. Time to look at analog circuits 101; how do you model a BJT transistor? I only ever knew as far as "if base voltage is high, then current flows more or less freely from collector to emitter", not quantitatively what that means as you vary voltages bit by bit.
So... now I'm reading about how external electric fields affect semiconductor band gaps. Obviously.