One of the images shared by a Pixel 3a owner showed a message from Google that was sent with the update indicating that it was meant to be disseminated internally only. The heading of this page read "CONFIDENTIAL INTERNAL ONLY." Expecting only Googlers to receive it, the company wrote that those receiving the update should not discuss it or comment on it publicly. In the parlance of the industry, when a company is using its own products or software, it is called "dogfooding." Google, as you might note, requested that feedback about the update be sent via email to Android-dogfood-discuss@Google.com.
Assuming that the OTA update won't change between now and its actual release, the only changes mentioned were the security patch and bugfixes. But we won't know for sure if there are any functional updates for the Pixel line until the July Pixel Update Bulletin is released. That won't happen until early next month unless Google messes up again.

Screenshots show that Google mistakenly sent the July Android security update to consumers
ncG1vNJzZmivp6x7sbTOp5yaqpWjrm%2BvzqZmp52nqHyUu8yeZImhqJq5bn%2FAZqawppWnwG6zxK1kraCVnr9ultSlsGatoJmutbGMnpirpKlir7p5zKKqrZmbmqyqsJBqbXJvYw%3D%3D