After hours of research I still cannot answer the question whether one can define custom interactions for an app with the google voice interaction api.
As an example I would like that my Android App can answer questions like
OK Google - what is my remaining balance in MyCustomApp?
As I understood from https://developers.google.com/voice-actions/custom-actions the current state is that no one can define custom actions anymore or is this information not up to date anymore? Thus we have to stick to the predefined system actions?