头文件如何像OpenGL。h工作

I understand that header files are processed prior to compilation of the remainder of the source file in which it is included to make the process of developing code easier. I also know that they allow working declarations. however, I don't see functions in use in the header file OpenGL.h like I do in all the tutorials I have been researching. The OpenGL.h is very obscure to me with #define extern. I don't know what is happening. for instanceI understand that header files are processed pr