This cannot be a rounded rectangle because those are not fully or "perfectly" rounded at the two ends. This is more like an elongated circle? A flat cylinder?
I searched a lot, but among all guides or pictures that identify and name different shapes, I couldn't get this particular shape. The closest I could get was a stretched ellipse.
There are only two processes to get this shape as far as I know: the first one is making a long rectangle and making its corner radius exactly half of the height. The second one is putting two circles a distance apart and adding a rectangle between those. So, are these something like "perfect rounded rectangles" or "rectangular circles"?