Ours typically have a bit in the middle that's usually grass behind a curb, then a lower raised bit with a bevelled curb that's all concrete surrounding that, or else just an area around the center-grassy-bit that's even with the rest of the pavement but marked off with "don't drive here" stripes. The ordinary lanes are outside both.

I assume those are affordances so buses (say, school buses), trucks towing trailers, and stuff like that, can actually use the roundabouts. They'd be damn tricky for them to navigate, otherwise.