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.