For those who don't use C: header files are more or less an interface description and (usually) don't include any code. The code is added from external objects during the second "linking" step.

Oh, I see: including a header file injects forward declarations into the calling file. Each c file gets compiled separately into object code unless you directly import them like the code above is doing. In that case it essentially dumps the full text of the file into the calling file, which causes serial compilation of a single object file.

Unless I'm mistaken that clears some stuff up for me. Thank you!