There is simply no way to pre-store any arbitrary long text because the possible combination is just tooo huge. It is like draw a bullseye where you arrow landed. You search any paragraph, it search existing index, if not found then save it and assign it a new index. Of course it may use some algorithms to optimize/decrease the space needed.

From it’s wiki page, …wikipedia.org/…/The_Library_of_Babel_(website) ‘’’ The website can generate all possible pages of 3200 characters and allows users to choose among about 10^4677 potential pages of books. ‘’’