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

We know that a liquid contained in a vessel exerts pressure at all points and in all directions. The pressure at a point in a liquid is same in all directions (upwards, downward, sideways). It increases with depth inside the liquid.

When a body, say a block of area of cross section A, is immersed in a liquid, as shown in figure, the pressure P₂ exerted upwards on the lower face of the block (which is at a greater depth) is more than the pressure P₁ exerted downwards on the upper face of the block (which is at lesser depth).

Thus, there is a difference in pressure (= P₂ - P₁ ) between the lower and upper faces of the block. Since, force = pressure x area, the difference in pressures due to liquid on the two faces of the block causes a net upward force (i.e., upthrust) = (P₂ - P₁) x A on the body.

However, the thrust F on the side walls of body get neutralized as they are equal in magnitude and opposite in direction.

Upthrust acts on the body in upward direction at the centre of buoyancy (i.e., the centre of gravity of the displaced liquid).