https://github.com/eapowertools/qs-governance-dashboard is the easiest UI tool for accessing this meta-data.
In brief, you're going to need to cycle through apps using the QES APIs. There is no logical connection between an app and its extensions (or data connection) using the QRS APIs.
Hope that helps
If you open the app in the standard client and go through all the sheets, you will see what extensions are loaded in the sources tab of the developer console:
Here you will se extensions, themes, widgets. You will also see extensions loaded by other extensions, which is possible but not very common.
This works well for one app, but for many apps you will need some kind of tool that scans the metadata.
Hope this helps