Google is now pushing Fuchsia OS for their first-generation Google Nest Hub, secretly. Moreover, you will not find much of a difference, other than the underlying kernel. Fuchsia uses Zircon kernel instead of Linux kernel like Android. Moreover, this new update is for the non-preview devices as of now.
The experience remains to be identical to that of the old OS of the Nest Hub. Not much of the changes have been noticed yet since Google is working on making the experience as smooth and identical as possible for the Hub. The only way you can get to know if your Hub is running new Fuchsia is from Settings>About Device.
Nothing more interesting is in the new update, other than the swapping of the OS underneath without the user noticing it.
Source: The Verge