i've actually literally seen this happening, not messaging or typing about a subject, but phones in the room, talking about something, and then mysteriously google starts pushing that shit

it's extremely fucking hostile behaviour, and so easily brushed off as paranoid delusion. but i know i didn't publish any such reference at the time and then it was appearing in advertising. i don't think they have even come close to admitting it except in their brags that "we know what you are going to do" made by Sergey Brin about 5 years ago.

yeah, that's how they know. buried in the google software is a voice recognition engine, and it always has access to the microphone.