The goal isn't usually to determine whether the function is the perfect optimal version of the function that could ever exist, if the package it integrates with the the best possible package out of the 4 mainstream options, or to become totally and intimately familiar with them to ensure it's as idiomatic as possible or whatever.

You're just making sure it works correctly and that you understand how. Not superficially, but thinking through it indeed. That the tests are covering it. It doesn't take that long.

What you're describing sounds closer to studying the Talmud than to reading and reviewing most code.

Like, the kind of stuff you're describing is not most code. And when it is, then you've got code that requires design documents where the approach is described in great detail. But again, as a reader you just read those design documents first. That's what they're there for, so other people don't have to waste time trying out all the false starts and dead ends and incorrect architectures. If the code needs this massive understanding, then that understanding needs to be documented. Fortunately, most functions don't need anything like that.