yandex

Yandex and BOSS are just two of many sources: https://duck.co/help/results/sources

Beside PR talk, technically it's this way:

Bing through Yahoo BOSS API (which isn't free anymore) is the main source (and Yandex is used instead of Bing for certain countries in Europe&Asia). All other sources are smaller sources that augment the main search results. DDG is at the moment a meta search engine.

There was a recent search engine startup with its own large scale crawler, Blekko. Though, recently IBM bought it for their Watson AI, as a knowledge base.

From what I understand, DDG also has its own crawler, but yes, it's mostly a meta search engine at the moment.