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There was once a programmer that wrote his own code
Of course the first programmer did, but everyone who came after just copied her work and tweaked it a bit to suit their needs.
Basically, yeah. Dennis Ritchie wrote the C compiler because he knew exactly what her wanted to use it for and the kinds of code that he wanted to write. Then he went on to write the book that everyone used to learn the language.
This is true of probably every language, library, framework, etc. The original designer writes it because he knows what he wants to do with it and does so. Then everyone else follows. People then add more features and provide demonstrations of how to use them, and others copy them. It is extremely hard to just look at an API and use that to figure out exactly which calls should be made and in what order. Everyone just reads from the examples and adapts them as needed.