I was wrong.
The search results provided by Mojeek come from its own index of web pages, created by crawling the web.[6][7][8]
So, ddg and brave also have own indices. But i dont think, that it is matter, anyway they send data to 3rd party services.
I should have specify, that they don't block only tor. They block malicious traffic.
Yes, Qwant uses bing api(like ddg).
Sorry, i have no much time to write docs. Would you like to contribute?
Ok, i'll add it. Thanks.
To tell the truth, only yacy and sightnet have own index(database). Others are meta-search engines. They work like proxy. It's called meta search. (https://en.wikipedia.org/wiki/Metasearch_engine)
But duckduckgo is not open source project.