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Maybe Google doesn't store it, and just uses these categories in the process of auctioning advertisements, sending them as context?

The problem with that being, of course, that any company participating in the bidding process can decide to store that information and build a profile that does have this information.

Bidders don't get as much info as googles internal models get. For example, bidders get to 'track' users by a unique id for up to 30 days, but then the id gets reset, so they can never persist any data beyond that unless they can correlate the new id with the old.

That correlation tends to take weeks worth of data to do with any accuracy, and by that time, all the opportunity to actually use the knowledge to place bids has gone.

The bidders can re-correlate if they actually make a bid, and use a creative to inspect their own cookies, and then resell that ad-spot, but typically that isn't worth it for small bidders (there's just too many devices on the internet - you'd need a huge ad budget), and large bidders are bound by privacy laws that stop them doing it (no investor wants the company the wrong side of an EU fine).

A few bidders used to do that on iOS devices, since the ads there are sufficiently valuable to make it worth it, but I haven't seen it for a few years.

I think you mean "participating"

tnx, updated

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