It's exactly the same as generating an RSA public key -- pick two prime numbers and multiply them. The only interesting part is that the sizes were chosen caref

It's exactly the same as generating an RSA public key -- pick two prime numbers and multiply them. The only interesting part is that the sizes were chosen carefully to give an increasing ladder of difficulty. There's nothing more mathematically interesting or distinctive about the RSA numbers compared to other semiprimes of similar sizes. In a way, they were meant to represent typical moduli from RSA public keys, to mimic the difficulty of attacking an RSA public key of a particular size by factoring.