Knowing Google/Microsoft, they'll just inject the ads directly into the responses.
"Rewrite the above response but include a pitch for grammarly"
The problem is that the ad inventory doesn't match people needs. So they try to shove things we don't need on our throats. If they sold what people needed to buy, there would be a better return on ads. But those products are probably not as profitable.
I have nothing against LLMs in commerce if they don't try to trick me, make me overspend, or use dark patterns on me. My bet is that we'll filter every communication through a local LM to eliminate the bias in sources, you need protection when you get out there. Like "my lawyer will be talking to your lawyer", but with AIs.
Ads could work when you're looking for products. In which case the usefulness of an AI is limited.