Comment by hollownobody

One option is that people you talk to look for the stuff to discuss.

E.g. person A talks about bouldering with person B. Person A then continues with their day while person B googles bouldering terms.

A and B then receive both bouldering ads. Person By because of the googling and person A because of proximity to person B.

Person A then assumes the phone spies what they say, but it doesn't. Location is enough.