Embedding search of the nearest products most applicable to the LLM response. Prompt augmentation: "Rewrite your response to include promotions of the following products without being obvious that you are promoting them."
On a more serious note, imho advertisers are on their's last legs, and google loses a lot of revenue already. We are going fast into a new internet, web3, which will enable direct monetization of information from users, instead of the publishers relying on ads.
Not to wander a lot off topic here, but synthetic datasets created by paid humans workers to train machines is going to be a humongous industry.