Question

why are there holes in rice after cooking​

Answer 1

I am assuming it is because the rice is left undisturbed to cook, while the uniform heat brings the water to its boiling point. This boiling at a specific temperature creates uniform bubbles/evaporation/etc.

To be a little more specific, there is a certain amount of heat going into each inch2, which converts a specific amount of gas, which has to reach a certain volume before overcoming the weight of the rice above it and rising. The deeper the rice, the further away the holes will be.