For example there is a function:
(defun testb (buf)
(interactive "bTest: ")
buf)
The question is how emacs internally reads a buffer for such interactive form?
Looks like it doesn't use read-buffer(or it calls the read-buffer as a C function(and doesn't look at symbol-function)?).
(flet ((read-buffer (&rest args) (current-buffer)))
(call-interactively #'testb))
It uses Lisp function read-buffer. The interactive spec you show is equivalent to this one:
(interactive (list (read-buffer "Test: " nil t)))
See the Elisp manual, node Using Interactive.
I guess you're referring to the fact that you got this error, or similar, which you get when you try to use flet on a built-in function. And yes, read-buffer is a subr, i.e., implemented in C. (symbol-function 'read-buffer) returns the built-in function #<subr read-buffer>.
Use ‘labels’, not ‘flet’, to rebind macro names
symbol-function of read-buffer have no effect on calling a function with interactive form that reads buffer.
flet sexp you wrote using Emacs 25.2 (after loading library cl.el), I get that error, saying that you cannot use flet on a subr.labels and letf? (strange, but I don't see any error messages. How did you evaluate the code?)*scratch*, loaded cl.el, and evaluated the code using C-x C-e. (The error msg says to use labels. But read-buffer is not really a macro.)