nof4n

CloudFlare: 94 Percent of Tor Traffic Is Malicious

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?

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)