Really? They have buried Cat5/6 cable speaking Ethernet coming onto your property?

I've never heard of that before. How does that work? Your ISP would always have to have some infrastructure within 100 meters of your router then.

In my case I have AT&T Fiber that IIUC carries ethernet frames encoded optically.

No. But fiber to the building and rj45 inside is common here. Or fiber to each apartment. All regular ethernet.