Computer systems don’t do true randomness, work arounds usually involve choosing a value that changes consistently as a seed for your random number generator, like an number from the clock.
From the individual perspective its going to seem like true RNG, but on a mathematically level, its not.
IDK if anyone has ever figured out what blizz uses for this, and im sure they have changed how it works over the years.
Classic is running vanilla in the legion client and players did figure out that character id on creation controlled how ony did threat. They patched it out since they prob didnt want an easy fight being easier.