Does anyone have kept an eye of who uses what back-end?

DDG used to be meta-search on top of Yahoo, which doesn't exist anymore. What do Gabriel and co-workers use now?

I think they all use Bing now.

Kagi is mostly stealing results from Google and disenshittifying them but mixes in other engines like Yandex and Mojeek and Bing.

DDG is Bing.