From their product page:

> Note that Brave Search is based on an independent index. However, for some queries, Brave can anonymously check our search results against third-party results, and mix them on the results page. This mixing is a means-to-an-end toward 100% independence.

Chromium was chosen for its compatibility with the Web today; we did some early testing with Gecko but it was clear that our best path forward would be paved in chrome That said, we patch as needed (for security and privacy purposes; see https://github.com/brave/brave-browser/wiki/Deviations-from-...).

With regards to 'Google results backfill', any competitor to Google has a lot of work to do if they wish to close the gap in terms of quality of results. Fallback Mixing (https://search.brave.com/help/google-fallback) and the Web Discovery Project (https://brave.com/privacy/browser/#web-discovery-project) are necessary if we are to slay the dragon, so to speak. By having an army of anonymous volunteers helping to train Brave Search, we (as Internet netizens) are mounting our most hopeful attack against the Google monarch.

As Brave Search grows, our independence (https://search.brave.com/help/independence) grows as well. And you couldn't be more right about the importance of different viewpoints in this space—check out https://brave.com/goggles/ for more on that.

I hope this helps!