I have now read enough applied circuit diagrams that I believe I'm missing something that is obvious to those skilled in the art: Where an abstract diagram has a few components, a complete wiring diagram of the same circuit is absolutely littered with capacitors in the microfarad range.
(To do: Find and insert some example images here.)
I can sort of understand how these capacitors would smooth out voltage drops, (and I guess there are usually a few diodes to deal with voltage spikes)?
But how do engineers come up with these applied circuits? Are there unwritten rules they just know, like, "Yep, gotta put a microfarad on this side of a diode, that side of a transistor, and three of them in parallel by this sort of switch..."? Or do they wire up a prototype, start scoping every junction, and add them based on actual signal noise they observe? Or is more prophylactic, e.g., "I wouldn't want a lot of voltage noise here, so why not add some capacitance just to be sure?"