I'm using Xcode 11.2.1.
Clicking the "+" in the top right (while a .storyboard file is open) gives you a list of objects you can add to the storyboard (views,...) but there are two more buttons in that pop-up window: The Media Library and the Color Library.
There are a lot of useful icons (trash, arrows, checkmarks, letters, numbers,...) in the Media Library by default and you can drag them to the Storyboard. But once I run my app, the icon is gone in the simulator (it's still there in the Storyboard).
It's also possible to add one of these icons to a button (Attributes Inspector > Image) but this makes the button unclickable in the simulator (the font turns white) and the icon is also gone again.
This might be a stupid question: How do I actually add images from the Media Library to my app and still have them show up in the simulator?