This article has insights from folks who actually tried: https://0x65.dev/blog/2019-12-06/building-a-search-engine-fr... TL;DR you need a lot of money and users (catch-22). Also, massive scale and massive noise. You can also search HN for 'cliqz' for discussions about Cliqz shutting down.
In case you weren't aware, Brave Search is led and built by key team members from Cliqz: https://search.brave.com/
The growth and usability of Brave Search this past year has been strong, with some significant features planned throughout 2022.