You’re half-right. The Library’s pages are indeed generated by a PRNG (pseudo-random number generator). Since computers can’t generate truly random numbers on their own, they use a PRNG. The way they work is by using a base number (a seed) to then generate more numbers using a mathematical formula (an algorithm). However, the Library produces pages which can be run through an inverted version of the PRNG to determine its original seed. This means that you can always go back to the same page in the future, and all other pages that you could ever find are in a predetermined location. The only thing you were wrong about is that the Library doesn’t store the page on its server by default.

The way a bookmark works is by saving the seed to a database. Simply put, think of a database as a spreadsheet. In column A is the name of the bookmark, and in column B is the page location (seed). When you revisit a bookmark, it finds it in column A, and then regenerates the page from the location stored in column B.