Psp, ds, pc and other machine's programming.?

This might be the most stupid question you have ever heard but i have a question. How do people figure out the programming language for psp or ds or something. I own a psp myself and people make homebrew for it (home-made games). How do they know how to make them How do they know what excactely to type to get a certain effect on a certain machine. Like on pc the command (in C or C++ i don't remember) is printf or cout. For psp it's pspDebugScreenPrintf . Also to get a pc (windows) program to run you have to do #include but for psp you have to do #include or something like that... I'm just curious. The more extensive the explanation, the better. I really want to know. Thx in advance.