Counterintuitively, the software for the Google Home Hub is completely different from the others. From 10/22/2018 Ars Technica article:
The Google Home Hub takes an odd departure from the other Google Smart Displays when it comes to software. While the interface is identical to the smart displays from Lenovo, JBL, and others, the base OS for the Home Hub is the Google Cast platform. Other Smart Displays were the launch platform for Android Things. So while something like the Lenovo Smart Display could be seen as a stripped-down smartphone, the Home Hub is more a souped-up Chromecast. My only guess as to why Google did this was to keep costs down. The Cast Platform should have lower system requirements than Android Things.