Tap the icon to open it up. It should expand afterwards to depict a slider and scale.
(Edit; I confused the above with the focal length slider.). Tapping the focus +/- should show a change in mm taking place and the screen image should alter to reflect that.
If you haven’t found this yet, when in auto focus you can tap the screen once on an area you want to use as the focus area and the focus tightens up on it.
It actually does work but you often will not note the change on the screen image. It shows up clearly in the video though. It’s something I frequently forget to make use of, which I regret afterwards.