I mean, I'm pretty sure we know our phones aren't listening. We have privacy controls for microphone access and people are checking this stuff pretty regularly

I mean, I'm pretty sure we know our phones aren't listening. We have privacy controls for microphone access and people are checking this stuff pretty regularly because they're scared of it too. If that video about flags appeared when you hadn't talked about flags would you have thought about it? Probably not. You only made a note of it because you'd talked about flags and it "confirmed" your suspicions about phones listening to us. What initially led you to talk about flags? Was flags something you'd actually looked at before? Maybe, but again you probably never thought anything of it and you probably can't remember whether you did or not. But youtube would remember. Advertisers also use so many different things about you. They estimate or know your age, interests, location, eating habits, etc.