They don’t share it directly with each other, it’s through data brokers. Oracle being the biggest (here in the states) along with Experian, equifax, etc. I’m not talking about platform specific data. I’m talking about browser history, cookies, transactional, and the like. I know for a fact that one FAANG uses your browsers history to run adtech against it to show you relevant ads related to a previous search. Just because you couldn’t access it, doesn’t mean someone else wasn’t.