[dr0verride]

The system knows to serve you ads about the new topic because it's new. You're already getting ads for the stuff you're normally talking about. The new topic stands out easily.

It doesn't have to be your phone. Could be your TV or any other device.

Most importantly there's just patterns of behavior. Companies are absolutely desperate for every scrap of data they can get on you. Why would they not capture audio from your mic?