I have the same question. My Pixel 3 has installed many updates over the past months and each time there's been zero trace of said updates online aside from a new image name on the changelog page. No patch notes, no info on the forums, no info on Reddit, and even clicking on the image's hyperlink gives me GitHub's 404 page. I still have no idea what my phone's actually been installing in the background.
Where can we find a changelog for this? I can't even find an option in Settings to disable automatic updates so I can't prevent these unknown updates from being installed.