Apple has (finally) approved the CARP Studies app and it is now released and published in the Apple App Store. Note that this is an unlisted app and hence cannot be found using search in the app store. This is deliberate, since we do not want non-invited users to download and install the app.
You can read more on the app’s overall design and the user tutorial to understand how it works.
The app is also available on the Google Play Store.