GoodNotes is a powerful tool for teaching and presentations. The external display only shows the content and hides all user interface elements.
Normally, when you zoom and move the page on your iPad, it will be reflected also in the external display. You can lock the page position and zoom scale in the external display by tapping on the lock button on the toolbar. When the external display is locked, you can zoom in to write while showing the audience an unzoomed page.