int num = *(int *)number; What does this do?

The (int *) part casts the variable number to a pointer to an int, then the * in front dereferences it to an int.

The function takes a void*, but somehow it knows (perhaps it's required in some documentation somewhere) that the pointer it's given actually points to an int.

So, (int*)number is "the original pointer, converted to an int* so that I can read an int from it", and *(int*)number is the int value that it points to.