What it is
Compound learning is what happens when a skill from one area quietly improves another. Drawing a mountain teaches spatial reasoning. Spatial reasoning helps on the guitar neck. The guitar teaches musical structure. Musical structure changes how you sketch an idea at work.
None of it was planned. It happens because the underlying abilities (pattern, shape, proportion, structure) don’t care what domain they’re in.
How it shows up for me
I used to feel guilty about my hobbies, like they were stealing from “real” work. Now I think they’re the cross-training. I suspect learning a few things side by side beats mastering one thing at a time, though I can’t prove it.