There is one property of all of mathematics that is really striking, and it’s most clearly visible in foundational theories, such as logic, category theory, and lambda calculus. All these theories are about composability. They all describe how to construct more complex things from simpler elements.
Another desultory attempt to learn category theory, but it continues to be the kind of thing I should be into rather than something I am actually into. Also another half-assed attempt to learn to play keyboards.
One of those things I feel I really should know more of. It's my kind of thing! Extreme abstraction! But I've failed to pick it up by osmosis, never felt that motivated. I do abstraction, in computer code, so not clear what a math theory of it buys me. If I find out, I will write it down!