I have experienced it, and in my experience it was in no way possible for it to be confirmation bias. My wife and I sometimes do something weird where we just talk for hours. Crazy I know, but we bounce ideas off each other to an extent that we get into conversations about stuff we have never talked about nor ever searched for. Not even anything remotely close to related to things we search for.

We used to have and use a lot of Google home minis. Within hours we would see ads for the exact thing we were talking about. We would see ads for thing related to conversations we had within hours. We started getting hyper vigilant about it. We started randomly talking clearly and loudly about nonsense subjects and products that we have no sense to talk about and waiting to see how long it would take to see a suggested ad pushed to us by Google. It usually took less than 24 hours no matter what it was. This went on for months.

It became a running joke to us and I would walk into the room and say something like "I would love to buy a farberware brand vegetable peeler. There is nothing more that I would like than to purchase farberware brand appliances and homewares". My wife would laugh, and usually before the end of the night I would have large targeted ads on my phone for farberware appliances.

Honestly since we stopped using the Google home minis (since they barely work with anything due to googles bullshit software support) it happens far far less.

One thing that they are confirmed to do is when someone you interact with a lot searches for something, that person may be interpreted as a family member and results from their searches may show up in your ads. Devious af.