Google Timeline claims (e.g. here) that "Your Timeline is private, so only you can see it."
What does that mean? As a cryptographer I would take that to mean that the Timeline data is encrypted using a key that only I have access to.
But based on some of the features, that doesn't seem likely. For instance it can be accessed from any device provided I log in to my Google account, which would make key management hard. It seems that the recommendations it sends are based on it doing some ML on the data, which would be very hard to do if it were encrypted. And it now sends an email with a customized summary of annual activity. (I don't actually have Timeline enabled; this information is based on what I've heard from friends and seen online.)
Is Google actually doing some fancy FHE or MPC thing to analyze Timeline data while it's encrypted? Is the data being processed by your local devices? Or by "only you" does Google mean "only you and Google"?