On my Motorola from 8 years ago, the voice command was "OK, Google" or any custom short phrase you trained for it. But I think it would be really hard to get a useful but concise list of keywords... It would certainly be possible to listen for, say, "new Subaru" and show an ad if that one thing was detected, but the point is that you want to select which one of a million ads to show you'd need far more keywords, which gets computationally expensive quickly. It would probably be more battery efficient to compress audio that sounds like speech aggressively and then send it up to The Cloud for analysis...scary stuff, and part of the reason I don't have any home assistants!