In Pages, Numbers, and Keynote 3.3 for iOS, Apple has done away with its custom file manager and instead uses the a version of the iOS 11 Files App to allow the user to choose and save files within the applications,
The Files-like document picker User Interface on Pages and Keynote 3.3 for iOS 11:
Is this a publicly available view controller that developers can implement in 3rd-Party Applications?
Apple mentions in a user support document that these apps "work together" to present this view. Link: https://support.apple.com/en-us/HT208077