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).

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)