Do we know whether any of this stuff is actually happening, in reality, to actual people, based on some IP address's history of clicking ads? Any concrete examples you can link to?
Data brokers get their information from all kinds of sources. There is no complete breakdown on where it all comes from in every instance that the data is used, part of the problem with surveillance capitalism is that there is zero transparency and near zero accountability, but yes, data brokers do collect your browsing history and that includes what ads you view/click
As for examples of that data being used "in reality, to actual people" you might find some good info in these links:
Employers and landlords using data brokers for hiring/rental decisions:
Keep in mind that this is a rapidly growing space.
Travel sites, retailers, even grocery stores have been looking into how to use this kind of data to set the prices of their goods on an individual basis to make sure that they can squeeze as much money out of you as possible. The main thing holding them back so far is that consumers view discriminatory pricing as unfair, but they've been working hard for a long time to change that view. If you happen to find a place that requires you to scan a QR code to see prices or get a menu, you might want to check with the people around you to make sure everyone is paying the same price.
