Thanks! Sure. We are 100% transparent on all the search providers we're using, you can see them in the FAQ, and Yandex isn't used.

Initially we called all search providers and merged the results in a round robin fashion (so first of the first provider, first of the second provider, first of the third provider, then second of the first provider, second of the second provider, and so on), deduplicating them, but this was becoming very costly and inefficient once we had 3 and more search providers (most providers will return results within 500ms, but not infrequently one would take up to 2s or more — we timeout there, so I don't know if it'd take much more —, slowing everything down), so now we give everyone the choice of which providers to use first, and we pick results from the first two (we're actually considering switching to just the first, as costs are still a bit high and we don't want to increase pricing).

I hope that provides some more clarity! Happy to answer any more questions.