I work in the search engine space. Pretty much everyone is either Google or Bing under the hood, including DuckDuckGo. Deals are made on ad revenue shares generally. If I recall correctly DuckDuckGo just monetizes on product affiliate links though.

Fun fact: DuckDuckGo used to use Yahoo, but Yahoo just has a deal with Bing. In other words, DuckDuckGo paid Yahoo, to pay Bing, to deliver Bing search results. :)