A question about the libraries and their code.

[SheeppOSU]

Libraries are always very helpful for various things, but I'm always very curious what the code is. I know that I could view it if I wanted, but it's more that I'm curious how the libraries can do things like interact with browsers and open up windows like in Pygame. What different code is combined in order to do this stuff? Are all libraries made up of regular Python code? Like I could recreate pygame or os if I was skilled enough? Thanks in advance for answering my questions.