Google originated its voice command with “Ok Google” since its inception.
The Google Voice Search application for many years has been listening for Android to the voice command “Ok Google.” This carried forward to the Google Assistant when it was launched in 2016. When the Google Home launched in late 2017, it listened to the “Hey Google” command as well as the old command. So now Google is rolling out the update of “Hey Google” command to the Android platform.
Back in October, Google began experimenting the “Hey Google” hotword on phones. At the time, it just operated on a handful of phones, but now it has actually begun turning out to most of the Android devices. A notification can be seen asking people to re-setup the voice to the new hotword, “Hey Google”.
Once you re-setup the voice model you can use either “Ok Google” or “Hey Google” to open Google Search/Assistant, or
You can also do it from Settings > Google > Search > Voice.
Just like with the old phrase, you can choose to use it even if your screen is off, like before.