why tea will have a upper surface like a layer after cooling sometime?

The layer is called milk skin

Will forms skin on top when heated because of chemical reaction that affects how protein and fat molecule interact with each other

When milk is heated rapidly, save the water in a TV operates from the surface. This exposes proteins and fat molecules, which Bandhan dry out as warming continues.

Skin most commonly forms when milk is heated over a stove top, stores are generally capable of reaching very high temperature is quite quickly, do it can happen in the microwave as well, it's generation can be prevented with constant steering.