I think I bought the Amazon FireTV in 2018 to replace my old Chromecast and after 3.5 years of services, it became a brick all of a sudden. Kodi has been great on the FireTV but it's not perfect. So it's time to try out the latest Chromecast Google TV, which is very much like the FireTV on which I can install apps like this. Of course external APK is always tricky on an Android system.
First, the old instructions for the older Chromecast Google TV (i.e., those before the "Sky Blue" version) for turning on the Developers option no longer works. Now you need to download an app called "Developers Tool" from Google's App store first (thanks to reddit) Then go to Settings->App->Security to allow installing APK from unknown source.
Second, install "Send Files to TV" app on both your phone and Chromecast Google TV. Use your phone to download the APK you want (like the one mentioned above) and send to Chromecast Google TV (thanks to this post)
Third, install a File Manager (like the one from the previous link)
Finally you can open the APK in the File Manager and install it!