I rented a Kia Seoul which displays a Yield icon on the dash when I enter a merge or lane drop.
Plus it displays a tiny speed limit sign at speed limit changes and even knows when a curve is approaching and displays a curve warning sign with the direction of the curve.
It can’t be radar as you would need a fixed object to fixate on. So I’m guessing it has a camera with certain signs programmed into the computer that is activated when certain signs are caught by the camera and makes a match.