so this really does happen then? Because I used to be convinced it wasn't a coincidence when I saw ads online for some niche uncommon topic I had recently talked out loud about.
It's currently not economically possible to listen to user's conversations, transcribe them to text, and serve ads based on that. It would cost orders of magnitude more in processing power than you could get from the extra sales.
This might change in the future, of course
Google had (and has afaik) significant challenges implementing multiple wake-word detection for precisely this reason.
Transcribing a couple of words accurately on-device without a major performance penalty (so that it can be running in the background always) is just _barely_ coming out now.
Of course running it 24/7 in the background would ruin my battery, you would have to be smarter than that.
Anedoctally I belive Meta does something like that because I consistently get ads on Instagram about topics I talk with a friend on Whatsapp and sometimes that is done completely via audio messages. Though I might be wrong and leaked the topics in text messages among other possibilities.
I think it can be economically feasible. They can have a model optimized for their topics which can be orders of magnitude faster than general-purpose speech recognition. Low accuracy probably wouldn't be an issue as they are able to fine tune the user topics of interest via its interactions with the ads (e.g. click rate, time spent before scroll).