But there are absolutely rules on whether Google -- or anything else -- can use that search index to create a product that competes with the original content creators.
Google indexing of copyrighted works was considered "fair use" only because they only offered a few preview pages associated with each work. Google's web page excerpts and image thumbnails are widely believed to pass fair use under the same concept.
Now, let's say Google wants to integrate the content of multiple copyrighted works into an AI, and then give away or sell access to that AI which can spit out the content (paraphrased, in some capacity) of any copyrighted work it's ever seen. You'll even be able to ask it questions, like "What did Jeff Guin say about David Koresh's religious beliefs in his 2023 book, Waco?" and in all likelihood it will cough up a summary of Mr. Guinn's uniquely discovered research and journalism.
I don't think the legal questions there are settled at all.