Below and Under are the prepositions which basically mean ‘lower than’.

However, under may indicate contact between the two objects/surfaces mentioned in the statement. On the other hand, below indicates space between the two objects/surfaces. It is also used in measurements of temperature and height to mean ‘less than’. Under is used more than below.

He kept his gift under his pillow.
The toy car crashed under his feet.

There are huge rocks below the earth’s surface.
The room temperature is below 20 degrees.