Google brought Assistant to Chrome OS by way of the Pixelbook in 2017, and it also came on the Pixel Slate at launch. Now, more Chromebooks can access Assistant with both voice and typing input on Chrome 77. Although, Chromebooks tied to G Suite accounts won't have Assistant just yet.
You can enable Assistant on Chrome OS under Settings > Search and Assistant > Google Assistant. The Pixelbook has an Assistant button, but that's not the case with other Chromebooks. You can instead call up Assistant by holding "Search" and pressing "a." That lets you start typing to Assistant, but you can also use voice input with the "Hey Google" hotword.
Assistant on Chromebooks will do most of the same things you can do on a phone like playing music and exploring your calendar. There is, however, some Google Drive integration you don't get on other devices. You can ask Assistant to "Create a new document," and that's what you'll get. Assistant on a Chromebook can also open existing documents with a voice command. The stable Chrome 77 rollout will take a few weeks to wrap up, so don't worry if you don't have Assistant yet.