Is it possible to make the phone listen to the user in the background?

Question

Android

There is an idea for an application (no, not spying on a user), and I wonder how feasible it is.
Here is a case: the user enters the application, presses the "start" button, his phone asks something like "are you ready to provide your microphone" (until the user enters the application and turns off this function)
(as for example with geolocation when the user can provide it)
And all this time it will listen to the user's application in the background
Actually, is it possible to implement this? If on Android I still believe in this, then with iOS there are doubts

Answer 1

OK google constantly listens until it hears the passphrase.