On its Android Developer blog, Google has announced that its built-in voice search now works with third-party apps. For example, if an Android user says "Ok Google, search for hotels in Maui on TripAdvisor," the voice search feature will be able to perform the requested task in the app.
However, in order to enable this capability, mobile development firms need to add a few lines of code to their apps. The details are on the developer blog post. In addition, the users need to be running Android 4.1 (Jelly Bean) or later, and they need to enable the "OK, Google" detection from any screen in their settings.