Azygos vein -
Azygos vein is formed by

Subcostal veins
Ascending lumbar veins
Lumbar Azygos veins

It crosses the diaphragm on the right side, by piercing the right crus or the aortic opening.
At the level of T4 vertebra, it arches over the hilum of right lung.
It opens into the superior vena cava.
Formed by arch of aorta
Opens into inferior vena cava
Arches the groove of the left lung
Ascends through the aortic opening in the diaphragm