For me, its less about ads (I run adblocks everywhere, both on the computer and at the router level); but, the same idea surfaces in YouTube recommendations.
One recent example was some random show a friend was telling me to watch. Later that night, YouTube started recommending videos about the show. I've tried to explain it logically, in much the same way a character in scooby-doo would say "there must be a logical explanation for all of this" when faced with a ghost.
Did I search for it, or otherwise leave any intentional digital footprint of interest? Absolutely not.
Its reasonable to think Google has correlated the two of us as knowing one another. At the very least: I have this friend's phone number and email address in my Google Contacts. Our phones were in the same room together, and I certainly have some Google Apps installed. Maybe they know via GPS, or maybe its same WiFi/IP Address. It is reasonable that my friend has expressed a digital footprint of interest in the show, and Google is now funneling the results over to me, thinking "hey you have friends who are into this obscure show, maybe you will be to"
Of course, Google does not communicate its reasons. Its a black box.
What I cannot explain is why it chose that night to make the recommendation. We spend a few hours every week together. My friend has been watching this show for a while, mentioning it maybe once previously on a communication channel I feel reasonably satisfied as being "secure". Then, they mention it in person, and Google chooses that day to make the recommendation to me. Its one thing to assume I may be interested because this friend is interested; what raises concern with me was the perfect timing.
The funny part is, maybe Google isn't listening. It literally doesn't matter. Everyone I talk to, in my bubble outside of tech, believes they (meaning, "Big Tech") are. So, either: Google should be listening, because by-and-large people seem to think they are, and still use their products. Or: Google needs to realize their black box magic is conveying the impression that they are listening, and if their goal is to build trust, saying they aren't listening isn't enough; they need to dumb down their algorithms, or at least explain it better.
Personally: I think they're listening. I suspect its unintentional. I suspect their systems have grown so big, with so little responsible oversight, that some data stream is getting fed into some classifier then into another algorithm, and in the end its influencing the profile it builds on google accounts for things like ads and recommendations. I suspect it isn't the smartphones doing it; I suspect its Google Home speakers, in combination with a less controversial temporal + geographical correlation between two people.
Sure, this all sounds crazy. If you reading this works at Google; prove me wrong. I want to be proven wrong, with emphasis on the word proven. Why does YouTube make the recommendations it does? Why doesn't it explain the signals it uses in the generation of a recommendation? It must either be because you're afraid it would expose the depth and breadth of signals at your disposal, or, the algorithm isn't sophisticated enough to relay that information. And, maybe, just maybe, you'll begin to realize how bad both of those options are.