https://www.cnet.com/tech/tech-industry/apple-unblocks-googl...
But Twitter had been tracking apps installed on a users iPhone until Apple restricted access to the API that they used.
https://www.cnet.com/tech/mobile/twitter-is-now-tracking-the...
The purpose of the API was for one app to send messages to another app. But it could be used to tell if an app was installed.
A data science company I used to work for got hired in 2017 by a large American telco to handle this exact same sort of data coming from antenna location to do better ad targeting.
The reason why Verizon or AT&T do not have the ad capabilities of Google or Meta is because they are giant incompetent corporations that are incapable of developing anything in any area that didn't exist in the 1980s.
There is some obsession amongst a subset of techies with knowing everything, and that extends to the daily minutiae of the lives of others.