Couple of months ago, Google begins the rolling out of the ability to activate Search or Assistant on phones by saying “Hey, Google” but this was limited to some phones. Now, the command will now be available for more phones.
For the new setting to work, you will have to re-setup the voice model. You will get a notification asking the same should appear on your Android phone (if haven’t already). Manually, you can do this by heading to Settings -> Google -> Voice.
The moment you are done, you will be able to use both “Okay, Google” and “Hey, Google” to perform Google searches or call the Assistant.