Developers can use iMovie for Mac to create the App Preview videos that you see in the App Store. It isn’t a new feature, but until version 10.1.9 was released today, the feature did not support the iPhone X or certain iPad sizes.
Other video editing apps can be used to create App Previews, but iMovie is convenient because it automatically configures export settings to meet the requirements of App Previews. The trouble is, releasing an update to support the iPhone X that was released five months ago is too late. To make the feature useful to Apple’s best developers who support new hardware as soon as it’s released, these sorts of updates should coincide the debut of new iOS devices.
Version 10.1.9 of iMovie also includes numerous bug fixes.