In general, just Google has access to your search history and advertisers ask Google to target say 18 year olds with a sugar affinity for their Coca-Cola ads but Coke isn't actually told who you are.

If Google is forced to allow a competitive marketplace for placement of ads then other companies need access to that data in order to provide targeting.

There are a bunch of holes everywhere when you dig into the details but that's the gist. A large hole is the third-party cookies which allow a lot more than Google to track you. Another hole is some advertisers aren't really advertisers and just plug into the ecosystem in order to learn data about people they could potentially advertise to.

Who also knows if we'd get real privacy laws if it become really common knowledge how much your devices track you and how easy that data gets shared around.