Why and how do these lead to the different sexes?

The chromosomes do not always determine sex. In some reptiles, for example, such as turtles and crocodiles, it is the incubation temperature that determine gender. These animals do not have sex chromosomes. When an embryo is developing, it is the hormones that determine sex. Male hormones would stimulate the development of male sex organs, and female hormones would stimulate the development of female sex organs. The temperature during incubation in turn determines which hormones are more abundant and active.

The X and Y chromosomes of humans used to be the same length and contain the same number of genes on them. Over time, however, the Y chromosome got shorter and it lacks many genes (such as those for detecting colors) found on the X chromosome. The difference in chromosomes in humans and other mammals determine which hormones are active during embryonic development, and that in turn determines the gender of the embryo.

genes are like recipes. Apparently the Y-chromosome usually has all the recipes needed to turn a foetus into a boy.

What will happen if we put 2 Y chromosomes together?

Unless there is an X-chromosome too, there will not be any new life (assuming we're talking about humans here)

you need to run that 2Y concept through a punnet square...