How to calculate the current rating of different metal part (copper and brass) used inside a switch/socket/plug?
You start with the resistance of the current-carrying parts. At what rate is heat produced by a given current flowing through a given distance through a material of a given cross-sectional area and having a given resistivity? How much heat is produced by the resistance of the connection? How does the heat flow through the material at what rate is it dissipated into the surrounding air? At what rate does the air carry the heat away from the product? What is the resulting steady-state temperature of the product? What is the effect does that temperature have on the material? You also must consider that heat is transferred to the insulating material that the current-carrying parts are mounted to. The same analysis must be performed for those parts.

To learn how to do that, you start with the study of physics, chemistry, thermodynamics, and heat-transfer. Then you move on to materials science and perhaps some other specialized fields of study. A generalized course in approaches to engineering problems would also help.