Until now, Google Assistant has been limited to the Google Home and the Pixel phones (unless you edited your phone's build.prop file). Assistant is slowly making its way to more devices, starting with Android TV. "In the coming months," it will become available on all Android TV devices in the US running Android 6.0 Marshmallow or higher (sorry, ADT-1 owners).
Assistant on Android TV is similar to Google Home paired with a Chromecast, allowing you to play content from supported services (like YouTube, Netflix, Google Photos, etc) with a simple voice command. The recently-announced SHIELD TV refresh will be the first Android TV with hands-free Assistant, allowing you to issue 'OK Google' commands as long as the SHIELD's remote can hear you.
In addition to the upcoming Android TV support, Google shared new details about Actions on Google. If you don't recall, Actions on Google is the API that allows developers to add their own commands to Google Assistant. Google has worked with Daimler and Hyundai on custom commands for Assistant, for example, "Ok Google, ask Mercedes how much gas is in the tank." More details will come from the respective manufacturers this week at CES.
It's nice to see Google Assistant arriving on more devices, but Google has a lot of work ahead of them to catch up to Alexa. Countless devices on the CES show floor have Alexa built-in, from smart speakers to refrigerators.