the health app on my iPhone is missing. How do I download this app?

Health app is one of few only that you cannot delete from iPhone. If you cannot find it on Home Screen, it means you have deleted the app icon from the Home Screen or your device has some restrictions on that prevents access to the app.

While on Home Screen, swipe left to right until you get to the last screen called App Library, then on App Library screen search for Health app and when you find it tap and drag the app icon from the search results list to the Home Screen.