I've always tended to think that it's not actually using the "listening" capability, but it's a coincidence people overlook. People search for things, their spouses search, then it pops up. One time I had a text conversation about tire wear, then I got a tire ad.
But I've been seeing more evidence that it has to be using the speech it's evaluating for voice commands. I had assumed that - in a civil society- nobody's using your discarded sampled speech.
I guarantee that I didn't text or search Miller beer. However, I did look at an article ranking the top craft beer companies. And I may be a target for beer ads. For all I know, I may have been getting Miller ads every week for the past 5 years, but I didn't think about it until I was given a Miller.
It's like when you buy a certain car; you suddenly notice every other one on the road, but you didn't care before.