I assumed the grandparent meant how do "you" track across your own properties..

For the likes of google and co, I wouldn't be surprised if we start seeing more ad companies requiring you to send some other PII via the api so they can turn a random tracking ID into an email address or whatever though.

The same user on a.com and b.com get different ID's, but a.com and b.com both send data to tracker.com which maps that ID to an email address and then tracker.com can easily combine 'em. Not sure it's legal to do so, when I was working in this space we were quite forbidden from mixing up tracking information from various properties