Brave uses its own index. It used to be supplemented with results from other engines but I believe they have now phased that out.

Brave is the best of the free options in my experience, and it supports "bangs" which let's you send your querry to a different engine (typing "how far is it to the sun !g" will pass the search to google. Duckduckgo also supports bangs), this is especially helpful for image searches (!gi for google images) since braves image search sucks dogshit 😅

Quant seemed like the second best free option in my experience. Some people don't like brave as a company for various reasons, so quant may be a good option for those folks. Its my understanding that Mozilla has worked with quant in some way, which is kinda neat.

Both have their own index making them a sustainable/viable option going forward, where meta search engines that use other engine's results are at the whim of those they fetch the results from (but may provide better results by piggybacking off a larger successful engine)