Independent index, but still dependent on Bing for some results.
https://www.theregister.com/2021/10/20/brave_search/:
> The company said in June that "for some features, like searching for images, Brave Search will fetch results from Microsoft Bing,"
https://search.brave.com/help/independence:
> Third-party providers
> Microsoft Bing is a third-party provider to Brave Search.