My point being, that if you have a large enough data set then you can find all kinds of coincidences within that set. I.e., I'm sure you've heard of the comparisons between Lincoln and Kennedy (i.e., Lincoln was killed in the Ford theater, and Kennedy was killed in a Ford). This is also known as the Texas Sharpshooter's Fallacy.

Sure, but does your friend google those things at any point (before or after your conversation)? Do you ever connect to the same wifi access points? Are your phones ever in physical proximity?

Almost certainly your answer would be "yes" to most of those questions, which also explains how you get the suggestions about things you "only talked about" with your friend.

Not a coincidence, but it is rather simple, really. If there is a user A who googles topic X, and there is another user B who appears to be their "social link" (via device proximity, occasionally utilizing same wifi access points, sending emails to each other, etc), it makes sense to suggest topic X to user B.

I've read the other good responses to what you've said, but nobody said this:

it is very easy to become part of a trend or fad without realizing it. Like you could have an avocado toast at your local cafe and fall in love with avocado toasts without realizing that the cafe is already part of a national avocado toast trend. So you only talked about that avocado toast with one friend on your weekly wilderness hike, yet somehow google suggested you might like avocado toast.

mentioning it because in this case there is no direct link, but it isn't coincidence either. So many things I've fallen in love with and taken up turned out to take off in a trend, but I didn't cause the trend nor notice that's how I heard about it.