[mkl]

DDG does have their own index, but also use Bing and many other sources. See the CEO's numerous comments to that effect: https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu..., https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

[Kiro]

Only for things such as widgets, not regular search results.