First off, it's in the process of being rolled out to even older phones, the ones that still run Lollipop. This rollout is ongoing for users with the language set to English in the US, UK, India, Australia, Canada, and Singapore, as well as in Spanish in the US, Mexico, and Spain. You'll get it if you're in Italy, Japan, Germany, Brazil, and Korea too. Once you receive the relevant (yet invisible) Google Play Services update, you have to opt-in, and then you'll have an Assistant app icon in your drawer.
The Assistant coming to tablets is probably the bigger news of the day, even if Android slates are slowly disappearing from the market. For starters, only tablet users with the language set to English who are in the US will get it. Hopefully it will make its way to more tablets in more languages in more places soon, but Google hasn't announced anything regarding that yet.
Despite now being available for phones on Lollipop, if you have a tablet it will only work if it runs Nougat or Marshmallow.