Question

When I am working with multiple frames, I want to decide in which frame Emacs should open files/buffers.

Answer 1

You can’t do that out of the box.

There are find-buffer-other-frame and the like but they open a new frame.

What you can do is write your own function like this:

(defun find-file-in-frame ()
  (interactive)
  (call-interactively 'select-frame-by-name)
  (call-interactively 'find-file))

This switches frame and then asks for the file, if you want to do it otherwise you have to do more work.

Edit: Here the version that asks in the current frame and opens the file in the other window:

(defun find-file-in-frame (noselect)
  (interactive "P")
  (let ((current-frame (selected-frame))
        (frame (completing-read "Frame: " (make-frame-names-alist)))
        (buffer (save-window-excursion
                  (call-interactively 'find-file))))
    (select-frame-set-input-focus (assoc-default frame
                                                 (make-frame-names-alist)
                                                 nil current-frame))
    (switch-to-buffer buffer)
    (when noselect
      (select-frame-set-input-focus current-frame))))