Google is bringing some changes to the Pixel Launcher, the launcher that is used in the Pixel phones that will add more clearly defined shelf and shortcut to launch Assistant. All of this will comes with its new bubble design that will be on the search bar and Assistant button.
This new design was seen in the Android P Developer Preview 1, which showed the new design on the search bar, wherein the previous designs, the search bar and the microphone button is connected. Now, they are separating the search and mic button to make it more obvious to users that one will provide a Google Search and the other will activate Assistant.