Where exactly are the inherent WordPress Functions defined inside or ouside the core of WordPress?

If we look at http://codex.wordpress.org/Function_Reference; we see that there are core functions listed for WordPress.The first two sentences say:

"The files of WordPress define many useful PHP functions. Some of the functions, known as Template Tags, are defined especially for use in WordPress Themes."

So my question is, where are these functions defined for the core and how does PHP recognize these functions in the first place. I tried looking everywhere but I could not find the answer.

Thanks in advance.