Google Assistant launched as a Pixel exclusive last year. The only other way you could use it was via the Allo app if you did not want to go the hard way which involves rooting. Fast forward to present and Google Assistant as gone from being a Pixel exclusive to launching with a bunch of other devices and is now even rolling out to devices running Android Marshmallow and above.
The feature is coming as an OTA update to Google Play Services so it is not carrier or manufacturer dependent. But because of the high number of android devices, it might take a while to reach your device. However, if you own a OnePlus 3 the update should have hit your phone or may do so anytime.
One OnePlus 3 owner has already received the update and has even provided screenshots. Google Assistant is like an updated version of Google Now but with added support for home control, shopping functions, and lots more.