Back in the day you'd read reference manuals which would tell you about the library, they might be hand written or a listing of the addresses and names. Today in the modern day, we still use reference manuals for writing C and C++. It's just encoded in websites, e.g. https://pubs.opengroup.org/onlinepubs/9699919799/functions/c... and https://en.cppreference.com/

But there are still libraries where familiarising yourself with the header is important, for example stb's libraries. man(1) and GNU Info were both invented to be computer-stored reference manuals, and indeed to this day you can type `man 3p open` to get the POSIX documentation of the system up in front of you.

I am not aware of many codebases that have manuals as complete as this, but back in the day (1970s-era to the 1990s-era) there would be internal documentation written as par of course for the specification created by the waterfall model of development. Post-2010 companies are very lackadaisical about documenting internal interfaces and libraries because they come from startup cultures where "move fast and break things" is the name of the game.