Mostly just boils down to a Chain-of-Responsibility pattern which is incredibly easy to reason about. Just a way to compose functions.

As far as digging into internal, how often do you look into Express or Rails internals or any other? For me it’s a rare day to need to do so.

Only time I recall having to read Spring source code was when I used their Social Media Auth library sometime in the early 2010s when it was quite a primitive experience.