I have a few "rare books" and have read many, you'd be suprised at what is publicly available on google books since like ~2010ish.
The first was destructive. This was for mainstream books currently being published so they had no value. It's (I believe) where you cut off the spine and scan the pages.
For rarer books, there was a non-destructive process. Basically the book was opened to each page and scanned. This was slower but didn't destroy the book.
I don't understand why these companies haven't just licensed the scans Google has already done. Why is each company doing this rather than just scanning the books once and sharing the scans?
Because for the vast majority of the books, Google doesn't have the legal right to license those scans. It would be legal if the books were out of copyright, but despite the connotation of "rare books" those generally aren't the books we're talking about. Further, in the cases where Google didn't destroy the original of the book they scanned, their scanned copy may be considered infringing under the new standard, so Google doesn't want call undue attention to what they have.
It was a distinction between library books and others. They partnered with libraries, and obviously, libraries didn't want their books destroyed, so they devised a non-destructive book scanner. Some of the books they scanned were indeed rare, but rare or not, you don't destroy books you borrowed from a library!
https://nitter.net/internetarchive/status/135809098218971955...
6 Feb 2021
At the Internet Archive, this is how we digitize a book.
We never destroy a book by cutting off its binding. Instead, we digitize it the hard way--one page at a time.