Question

When emacs do the grep, it can split the window and show a nice buffer , and Emacs has the function (grep args),I want to know, How can I get the source code?

Answer 1

Emacs executes the external grep process, and formats the output. Assuming you mean the source for the code that wraps that up and displays it, not grep itself, that is pretty simple:

M-x find-function <RET> grep <RET>

(find-function FUNCTION)

Find the definition of the FUNCTION near point.

Finds the source file containing the definition of the function
near point (selected by `function-called-at-point') in a buffer and
places point before the definition.
Set mark before moving, if the buffer already existed.

You may also find find-library useful to poke around with.