>well designed codebases

One thing to keep in mind is that what was well-designed 30, 20 or 10 years ago may not be considered such now. Hardware changes and so do the design decisions involving performance.

For example, if you are looking at C++ networking libraries, learning from ACE or even Asio may not be the best idea - better look at "thread per core, share nothing" Seastar[0].

Another thing is that it may be better to read design docs, not the code. For example, the rationale for the mold linker design[1].

[0] https://docs.seastar.io/master/tutorial.html#asynchronous-pr...

[1] https://github.com/rui314/mold/blob/main/docs/design.md