How was this burger cooked?

[T.G]

Juicy in beef is often thought of along the lines of water, but water isn't exactly known for it's flavor, so if it tasted good, it's fat. A high fat content in the meat will do that - keep it moist and then when you bite into it, the juices flow. 75%-79% lean ground beef will gush when packed and cooked on a griddle like that. Flavor could also have been made more "beefy" by using ground chuck rather than ground "beef" (could be anything - trimmings usually) or even seriously amped up by using ground brisket.

A dome lid might have been used to trap the heat to melt the cheese, not sure what effect, if any, it would have on the meat for such a short period of time.

Another "trick" for uber-juicy burgers, although I'm not sure too many restaurants would go through the trouble to do it, would be to stick a thick slice of salted butter in the center of the patty when hand packing it. Oy. Yum. Almost as good as bacon.

Can't say for certain how they cooked it, but there's my 2-yen.