The Google Assistant is (I'm sorry; was) an exclusive personal assistant for Google Pixel users. But, Google has started rolling out this awesome and helpful assistant for many Android phones running stock Android 6.0+. And, if you're a Moto G4 Plus owner, you'll know what I'm talking about.
Earlier, I uploaded a video showing how to get the Google Assistant on rooted Android phones. But, that might be of no use now, as here's a method to get the Google Assistant any Android phone running Android Marshmallow or higher.
1. Go to your system settings and search for Language and Input. Here, make sure that your language is set to English (US). Also, check if your current keyboard's language is set to English (US).
2. Go to Settings > Apps and click on Google. Now choose "Manage space" and then click on "Clear all data".
3. Now search for Google Play Services on Google Chrome. Then, tap on the Google Play services icon to visit Google Play Services on the Play Store. Now, scroll down and click on "I'm In" to sign up for the beta program. This will take a few minutes.
4. Now search for Google on the Play Store. Here too, scroll down and click on "I'm In" to sign up for the beta program. Wait for a few minutes, and you'll receive an update for Google and Google Play Services.
5. Update the two applications and just press and hold the home button to enjoy the Google Assistant.