I observed a clean experiment that showed a friend’s Google Pixel phone listening to us and adjusting news stories on Google app’s home screen.
However:
— IIRC the phone was unlocked,
— this only affected the news feed, and
— this was 5–6 years ago.
We 1) noted how Google app shows some selection of news after opening, 2) talked clearly for a minute about a very random and conspicuous topic in presence of the unlocked phone, and 3) demonstrated that the Google app showing an article relevant to the topic within a few minutes. The article was a few days old, too, so it was clearly boosted out of more recent stories.
The only reason it could be something other than the phone microphone is if I was misled by my friend steering us towards a predefined topic. However, that would require some extensive preparation to rule out the story appearing in the first step and would be very atypical for that person.
I recall seeing an article about Google admitting this and changing their policy to stop, but can’t seem to find it now. I imagine it was bad publicity, though to my friend it was a feature to see personalized content.