I found this from March on Wired:
Eich says Brave isn’t starting its search engine or index from scratch and won’t be using indexes from Bing or other tech firms. Instead Brave has purchased Tailcat, an offshoot of German search engine Cliqz, which was owned by Hubert Burda Media and closed down last year. The purchase includes an index of the web that’s been created by Tailcat and the technology that powers it. Eich says that some users will be given the ability to opt-in to anonymous data collection to help fine-tune search results.
“What Tailcat does is it looks at a query log and a click log anonymously,” Eich says. “These allow it to build an index, which Tailcat has done and already did at Cliqz, and it's getting bigger.” He admits that the index will not be anywhere near as deep as Google’s but that the top results it surfaces are largely the same.
https://www.wired.com/story/privacy-first-browser-brave-launching-search-engine/I tried Cliqz when it was out. I was not impressed and it looked like they were copying Google's listings for page 1.
So the index is only going to be like 1st page SERP results for popular terms. Shallow. They definitely need backfill and I don't blame them for not using Bing for backfill because every man and his dog are using Bing.
I don't have a demographic on who uses Brave browser, but I can't see an index that shallow flying for anybody but newbies. Plus there is the question of how long can they afford Google backfill and or how long will G provide it?
It is decent of them to provide a Mojeek link.