This article seems to have more details but I'm still a little lost in it all. They actually link to a sample "push page" in that post:

https://brave.com/wp-content/uploads/files_2019-9-2/sample_p...

Ah, actually, this explains things a bit better:

https://brave.com/wp-content/uploads/sequence.pdf

So there's a "realtime bidding system" where when the user access a page, google goes out to bidders in realtime to figure out which ad to show. I haven't quite figured out the rest but in the process somehow, those networks are able subsequently match up details about the user.

EDIT from the Brave article:

>>> Every time a person visits a website that uses RTB, data about them is broadcast to tens or hundreds of tracking companies, who let advertisers compete for the opportunity to show them an ad. The data can include the category of what they are reading – which can reveal their sexual orientation,[4] political views,[5] their religion,[6] and health conditions including AIDS,[7] STDs,[8] and depression.[9] It includes what the person is reading, watching, and listening to. It includes their location. And it includes unique, pseudonymous ID codes that are specific to that person,[10] so that all of this data can be tied to you, continually, over time.

EDIT why does this article keep getting flagged? Can it just be redirected to the Brave post instead?