This comment is made a lot but it's only half true. DDG does use Bing to source some of it's results but it does have it's own crawlers too. In practice this means that it isn't "effectively a UI over Bing".
Seems people can't find your source, so I did some digging and found this:
"We also of course have more traditional links in the search results, which we also source from a variety of partners, including Verizon Media (formerly Yahoo) and Bing."[0]
As siblings have covered - it is a proxy for some search results.
That's ok though - a privacy respecting proxy to a decent search engine is a useful thing. You get all the search results and they don't get to track you.
I don't think I've ever seen DDG show up in my logs, and I found this article from a web host that listed out their top crawler traffic and DDG doesn't even register (but something called Sogou is): https://deviceatlas.com/blog/most-active-bots-and-crawlers-w...
It might be that DDG sticks to the to X thousand sites or something like that, so I would imagine that Bing must be providing the majority of the index, but maybe not for more popular queries.
My guess is DDG mostly depends on bing technology, at least for anything that is non trivial. There is a lot of smoke and mirrors on what actually DDG is, i would be happy to learn about details of their deals, technology, etc.
A service that combined yandex and google would probably cancel out most of the worlds censorship. I'm not sure where you would be allowed to run such a service from though...