I think people greatly underestimate (or misunderstand) the pervasiveness of ad tracking pixels.

Basically any website that has ads or tries to sell you something has a tracking pixel. These pixels create profiles of devices and track almost everything you do while interacting with those sites.

These pixels don’t require any actual “information” about you, they’re only interested in what you (via the device you’re browsing on) will buy. They also don’t use cookies anymore, it’s usually a combination of user agent, IP address, and coarse location. As you said, companies will generally share these profiles.