They do a bit of their own crawling. They take other people's crawls; Google and Bing spend a lot of money on crawling and it's odd to try to compete against those.

DDG also take a lot of other people's crawls.

DDG then add a layer on top. For example, 'official sites' are identified.

(I was reading about this just this morning, and frustratingly I cannot find the post again.)