How does a brewery bottle prime?

Just being curious.. When I buy some my all time fav Bell's Two Hearted, how do the prime those bottles? Not priming sugar I'd assume. I'm drinking an Anchor Steam Liberty ale right now and reading the label "Entirely natural carbonation." I fully understand using DME instead of priming sugar, but I'd have a hard time believing Anchor Steam or Bell's or Sierra Nevada or Three Flolyd's or DogfishHead, etc., uses DME or Sugar to prime. Anyone know for sure?

There are a few breweries that will bottle before fermentation is finished, just within a couple of points. This is pretty dangerous for homebrewers because we don't always know exactly where ours is going to finish every time.