What is the cause of upthrust? At which point it can be considered to act?

Explanation

A liquid exerts pressure in all directions and at a point its magnitude is same in all directions. Pressure increases with the depth of the liquid.
When a body is immersed in liquid, pressure acts on it from all sides. Sidewise pressures are neutralized as they are equal in magnitude and opposite in direction. The pressure at the bottom face is not neutralised. This is the cause of the upthrust on the body.