Google isn't one to shy away from confusing branding changes, and the whole Google TV/Android TV thing is just the latest in a long line of examples. The new Google TV interface will come to third-party devices eventually, but the OS will still be called Android TV, at least for a little while. Right now, the new Chromecast with Google TV is the only device to officially support the upgraded UI, but there is a way to get it working on other Android TV hardware.
How-To Geek has a great explainer on this, but you need to be prepared to fiddle with developer settings and ADB commands if you want to get it running. Your ATV device will need to be on Android 9 Pie or higher for this to work. The Google TV interface is comprised of two parts, the Google TV Home launcher and a separate app that handles the search function (the latest version of the Google app pulled from the new hardware). How-To Geek has downloads for both of these, with the former coming from our sister site APK Mirror.
With both of those installed on your ATV hardware, you'll need to enable developer options and turn on network debugging so ADB commands can be run. You'll then need an app called Remote ADB Shell on your phone so you can send the commands to your ATV device. Head over to How-To Geek for exactly which commands you need to use, but it shouldn't take too much time to get Google TV up and running. The key is uninstalling the default Android TV launcher so it doesn't interfere with the new one.
The home button on your Android TV device's remote will now take you to the new homescreen and the search tab should also work as expected. Be warned, this isn't quite the full Google TV experience — it doesn't have the new settings menu and performance might not be as smooth as it should be. It's not bad as a stopgap until your device gets the full update, however, and it's certainly an upgrade over previous attempts to get this working on older hardware.
Use this method to make it easier to roll back
It's been helpfully pointed out in the comments that there is a way to disable the pre-existing launcher rather than uninstall it, which means if you decide to roll back to it you won't lose all of your customizations. The commands you need to use are as follows:
Disable: pm disable-user --user 0 com.google.android.tvlauncher
And to undo: pm enable com.google.android.tvlauncher
- How-To Geek