Google launched Assistant over a year ago with the original Pixel phones, but it expanded to more devices just a few months later. However, only phones with new-ish software were included. Starting now, Assistant is coming to more phones and (finally) tablets.
On devices that don't have Assistant, long-pressing the home button opens the chronically underutilized and neglected Now On Tap screen. Of course, it's called Screen Search now. That will be replaced by Assistant on phones running Lollipop, which were excluded from the initial rollout (it was Marshmallow and higher). You don't need to do anything—Assistant will appear in the next few days provided you are in the US (English, Spanish), UK, India (English), Australia, Canada, Spain, Mexico, Singapore (English), Italy, Japan, Germany, Brazil, or Korea.
Tablets have been deprived of Assistant all this time regardless of OS version. That's changing now, as we surmised from a recent teardown. However, it's only for Marshmallow and Nougat—no Lollipop. It's a bit odd Google doesn't mention Oreo, but the Pixel C is the only Oreo tablet right now. Presumably, that'll also get support. Assistant will hit Android tablets over the next week, but only in the US with English as the default language.