When you define functions in an interpreted language (as opposed to a compiled language). You have access to the file / string / text that defines the function.
In JavaScript for example you can read the definition body text of a function you have defined.
If you try to do the same for a function that is included by construction in JavaScript it is not implemented as text but as binary. There is no reason to show the binary code that implements that function because it is not readable and it might not even be available.
jQuery extends default JavaScript behaviour. It’s one of the reasons it was so highly appreciated and praised as opposed to Prototype.js for example. Prototype was altering the natural behaviour of JavaScript creating possible inconsistencies when using Prototype alongside some other piece of code that relied on normal functionality.
tl;dr:
jQuery extends JavaScript, there is functionality implemented using native code (which performance wise is a good thing).