You’ve probably gotten used to saying “Ok Google” to wake Google Assistant on your device but that doesn’t mean it gets any less awkward to say. Google has been testing out the hotword “Hey Google” on phones since October on a handful of devices. Now, Google seems to be bringing this to more devices. A notification has been showing up for more people prompting them to re-setup the voice part of Google Assistant so your phone can recognize your voice when you say “Hey Google.” Once you do this, your device will respond to either Ok Google or Hey Google when you want to launch Google Assistant or Google Search. You can also head to Settings > Phone > Voice model within the Google app to check if you have access to the feature already.
Source: Android Police