How does a Magnemite evolve if you never battle any other Magnemites? Same for Diglett.

How does Slowpoke evolve into Slowbro if it never encounters a Shellder?

If a Bannette is a childs doll brought to life by a grudge, why are there more than one of them?

If a Cubone wears the skull of its dead mother, why is it when you breed 2 Marowaks, the mother doesn't die, and the Cubone still has a skull?