Question

When the milk is getting boiled why there is a layer of fat?

Answer 1

Milk is an emulsion of an aqueous product (whey) and a non-aqueous product (curds). They will eventually separate, with the cream rising to the top due to it's lighter density. Heating it just speeds up the separation.

Answer 2

Because when the temperature is raised the high temprature seperates the creamy part of the milk from the rest of it; just like the effect of when oil and water is mixed together.