I really doubt they’re listening to your microphone. Constantly uploading your audio would be noticeable in bandwidth and constantly analyzing audio on device would kill your battery - at least currently.

What this demonstrates is how good tracking by other methods is getting. You don’t need to listen to someone’s microphone when you know what they and their friends/coworkers are looking up online and likely bringing up in conversation. It’s trivial to fingerprint someone and track near everything they’re looking up online, and even if you’re privacy conscious, many of those you associate with share their contact list with every app that asks for it. This makes suggesting things your friends are looking up pretty easy. Add a bit of confirmation bias to the mix and you’ve got this “listening to the microphone” theory, because you’re not counting the number of times an ad isn’t something you’ve been recently discussing.