They are shot with a dSLR and a very long lens. This one was likely shot with a 300mm lens. The camera is mounted on a programmable robot, and a mosaic of photos is created. They are then stitched together with software such as PTGui or kolor Autopano Giga. I've shot a number of images like this, in fact some of them over 4x bigger than this one. My largest one, of Prague, was released 2 weeks ago: http://360gigapixels.com/prague_gigapixel_panorama_900K_2018....

you can see more about my gigapixel images here: http://www.jeffrey-martin.com/gigapixel-photography