i personally think it starts with a seed, such as a specific color being in a corner (A LOT of times black is in a top corner and white in the opposite bottom)
or possibly a game thats a few moves in, but stuff is added/moved/removed. (Many challenges look like the start of a game but with random)

then it adds randomness, such as moving units or changing color/teir/unit

and sometimes it throws in a modifier unit (+1 or -1 king (when kings are changed its almost always more than 1)... or + a chaos portal or two)