User data mostly has short shelf life so what happens is API renting, not selling. That's what Google does via its ad exchange, which is fed by many signals but notably by search. Search ads also make Google the most money, but all their businesses use a single ad exchange.

Firefox has a default search deal with Google that makes most of their revenue. So does Safari (edit: the Safari deal of course does not make most of Apple's revenue, but it is rumored to be big, multiple $B/yr). These are how personal data flows to Google for big money back. (Chrome is worse: if you log into a Google account in any tab, then unless you opt out via your account settings, your navigation is tracked by the mothership.)

Brave doesn't have such a Google deal, and Brave Search won't collect personal or re-identifiable data.

Not sure, I don't use Firefox. I'd assume they don't, given that a lot of the privacy ethics in Brave carried over from Mozilla.