Serial Do Technobingo How Does A Serial Protected Computer Game Determine Which Serials Are Valid And Which Aren't?

How does a serial protected computer game determine which serials are valid and which aren't? - serial do technobingo

For example: Suppose you have two legal copies of a video game, in various series. If you installed the game, regardless of the standard they use, because they are legal. So the game does know the difference between valid code or just write a series of random numbers and letters?