> I don’t think Google has a secret back door tracking mechanism their ad network is able to use.
they have 80% population using search, youtube, storing browsing history etc while logged into google account, so they have lots of data about most of the people.
> you're the one who introduced the concept of it being secret
There is no publicly known method by which Google’s ad network is able to siphon extra data from Chrome users for targeting etc. If there is one, as OP’s friend suggested, definitionally it must be secret.
