Ask HN: Best codebases to study to learn software design?

Your own. Everyone else's is trash. Never understand someone else's code-- the whole point is that there should be just a handful of functions of yours I can call, and shouldn't ever care to see how the sausage is made. Otherwise, your abstraction sucks

I agree that the internals of my code should be abstracted away from my users. But don't I still want the internals to be well designed so they are maintainable and extendable?

The design will reveal itself to you the more you wrestle with the data and logic of the problem you are trying to solve. Full stop-- nobody on Earth would be more qualified than you at that point to say, unless they've already solved your problem