Your phone is technically always listening. Google Assistant is always active so it can pick up the 'wake words' it's programmed to recognize and start carrying out voice commands. This doesn't mean Google ignores everything else you say.

Your device listens in short snippets for the 'hotword', in this case, 'OK Google'. When the hotword is heard, that voice snippet along with your request is sent to Google for processing. If the hotword isn't heard, the snippet is deleted – apparently. We know that your Google Home is always listening.