Question

is a muscular sheet present below the lungs which helps to breathing in and out.

Answer 1

the muscular sheet present below the lungs which helps in breathing in and out or inhaling and exhaling for active breathing is known as diaphragm

Answer 2

yes, a macular sheet i.e. diaphragm is present which helps in breathing.
when diaphragm moves downward, the volume of chest cavity increases and pressure decreases so air moves from outside to inside (from high pressure to low pressure)
when diaphragm moves upward, volume of chest cavity decreases and pressure increases so air moves from inside to outside.

I hope it will help you