They do use their crawler to source some "smart" results, but the bulk of ordinary results seems to be from Bing.

Yep, DDG admists this (blog post from a few weeks back). When DDG first started, most of their results came from Yandex IIRC.

When I was looking into it I only found a few high level things like https://help.duckduckgo.com/duckduckgo-help-pages/results/so.... Nothing on how the sausage is made unfortunately. Same for Kagi which takes a similar layered approach (though anecdotally it seemed a bit more spread across backend sources).