Interestingly, the Pixel Update Bulletin states that there are no Pixel security patches with this update. However, all Pixel devices will receive a quarterly Android 9 update as part of this OTA that includes various functional updates and improvements. This also explains the change in the build numbers for Google devices from PQ1A to PQ2A.
So far, the only visual change that I’ve noticed with this update is that the “About phone” screen in Settings has moved out of the “System” section and been made its own section: