JC's approach is, I think, where you use code to lay out your UI elements. I have done that in the past, but for most of my work I am able to lay out the app so that the important parts are in the middle, and the bits near the edge may not be seen on some devices, but that doesn't matter.
The widest device currently is the iPhone X. It has a ratio of 19.5:9. The S9 phone is 18.5:9, nearly as wide. I recently had to update an app, and also add iPhone X support to it. Previously I had a stage of 1024x576, and extra bits above and below the stage that get revealed on iPad. With iPhone X the screen was so wide that it would eat into the 576 area, and that meant losing important elements.
So now I have a stage of 1024x576, and content that goes out to a width of 1280 and a height of 768. 1280/576 is a ratio of 20:9, so I have enough content to exceed what iPhone X needs.
For this particular approach you would use the default stage scale mode of "showAll".
Another thing to know is about Retina screens. If I really had content that was 1280x768, it would look jaggy on Retina and HiDPI screens. Anything that is vector works out ok, but for bitmaps I have 2X versions that I scale to 50%. Those look perfect on iPad Retina, and good enough on iPhone X.