Thank you! I try my hardest not to use Google for anything besides Gmail and YouTube and that popup always infuriated me. Had no idea you could turn it off.

I completely understand that - but it's still creepy, and it's a reminder of how big tech follows you around the internet. As soon as I see it, I choose to leave the website.

No, this is Google providing authentication with your Google account to third-party websites. If you are already logged into a Google account, it suggests that you log in with that account with a popup saying "Continue as <your name>" and shows your email and avatar.

It's an iframe so the website itself doesn't have access to your data, but this is not obvious just from looking at it - it looks like part of the site. To me it feels like it teaches users to trust websites in ways that make them susceptible to phishing.