I'm guessing one form it will take is simply by omission.

User asks for recommendation. AI generates answer saying product is absolute garbage. Company pays to simply have that portion of the answer just not appear. It will be a post-filter sentiment analysis on the original answer. Nobody can ever prove what would have appeared or not.

This is the beauty of AI - while a search engine is at least semi deterministic and you can reasonably question why it wouldn't bring up a site that is clearly relevant, AI has plausible deniability. who can ever say why it generates this answer or that?