[Elisp][Question] How to modify a list by index while preserving value outside of scope?

[Rodrigo Morales <moralesrodrigo1100@gmail.com>]

----
(defun my/f (foo bar)
  (princ (format "(foo: %s) (bar: %s)\n" foo bar))
  (cond
   (foo (setf (nth 0 bar) "100"))
   (t (my/f "apples" bar)
      (my/f "bananas" bar))))

(my/f nil (list 123))
----

---
(foo: nil) (bar: (123))
(foo: apples) (bar: (123))
(foo: bananas) (bar: (100))
---

I have some questions:

+ The second time my/f function is called (i.e. when "apples" is
  passed), `bar' equals `123'. The third time `my/f' is called
  (i.e. when "bananas" is passed), `bar' has a different value. We can
  conclude that the modification to `bar' in the second call affected
  the third call. How is this possible? `bar' is an argument of `my/f',
  as far as I'm concerned, any modification to a variable that is a
  function parameter only affects the scope of the function.
+ This is a minimal working example, in reality, the code I'm writing is
  more complex. In the code that I'm writing, `bar' is a list and I want
  to modify some of their elements by index. The only way I know is by
  using `(setf (nth index my-list) new-value)'. However, using this
  method seems to changes the value of the variable outside of the
  function call. Are there any other ways to modify a list by index
  without affecting its value outside of the function call?