My pet theory is that this is actually a geo-location type of tracking, combined with the recency illusion. Let's say person A was doing research on a product

My pet theory is that this is actually a geo-location type of tracking, combined with the recency illusion. Let's say person A was doing research on a product they're interested in buying. That product category is now associated with person A. Person A goes and visits person B. Because our phones track where we go by default for most people, now there's an association between Person A and Person B, and a potential likelihood that person B is also interested in that product. Thus the occasional ad shows up for the product Person A was researching. Since Person A and Person B are friends, it's possible Person A will talk about the product they're interested in. Person B now notices that they're getting ads for the product that Person A talked about.