Its really simple. Kind of like counting outs. Say you have KQ and you want to know how likely the other player is to have AA,KK,AK for some reason.

First we do AA. For someone to be dealt AA they have to be dealt one ace, and then another ace. There are 4 ways to be dealt one ace. There are then three ways to be dealt a remaining ace. So the number of ways of being dealt AA is 4x3 = 12. But AhAd is the same hand as AdAh so we divide by 2 because we've counted each hand twice. In other words there are 6 combinations of AA.

KK works the same way except we have a K already so there are 3 ways of being dealt the first K, and then 2 ways of being dealt the second K. Total combinations of KK are 3x2 /2 = 3.

Same again for AK. There are 4 aces and then 3 Kings. But this time we dont have to divide by 2 because we arent repeating any counts so there are 4x3 = 12 combos.

so in general a PP has 4x3/2 = 6 combos and an unpaired hand has 4x4 = 16 combos.