I've had a very similar experience.
Once discussed something, out of the blue, something I've never been curious about in my life, in the car, with a friend who also has never thought about the same thing.
Hours later we're both seeing related ads.
Now, I get that the amount of data required for such analysis is supposedly outside the bounds of what phones can do. But I can't see any other explanation. Neither of us ever searched anything in this subject, we talked about doe a couple minutes and moved on, never doing anything about it. We have very different interests, too.
Except I've had experiences that aren't explainable by alm this:
Discussing a random, never-thought-of-before idea with a friend, in the car. Neither of us had ever thought of this thing before (honestly don't recall now what it was). Discussed it for 2 minutes, then moved on.
Later we're both seeing related ads, yet neither of us searched for anything.
And it was something way out of left field for both of us, that neither of us had ever thought of before. The related ads were so jarring that we both told each other about it.
Oh, and my phone was rooted, de-googled (lineage), with heavy restrictions for the apps, no social media (I still don't have any accounts with any of them, except here), etc. The other phone was an iPhone.