[Reposting] How do independent programmers design their codebase ?

How do the programmers for big libraries like open-ai baselines or tensorflow, pytorch structure their code ? When I try to read their code, I see that they have split their code into various classes, utilized different data structures, structured their code into different files and sorted them in a particular order. Not to forget the extensive documentation they've provided for all functions with examples. When I start with a programming task, I somehow don't seem to think about all these factors. I focus only on the program executing like it should. How do I develop that skill ? Is this what is called production level code ? Are there classes I can take to learn ?

I have never worked in a company before. I've mostly spent my time in school writing academic code. Any insights will be appreciated. Thanks !