When Does the Buoyant Force Act?

Any object with a non-zero vertical depth will have different pressures on its top and bottom, with the pressure on the bottom being greater. This difference in pressure causes the upward buoyancy force.

At what point does the buoyant force act?

The exact point of application of the buoyant force is at the Center of Buoyancy. That being the center of the volume of displaced fluid by the object immersed in it. Gravity acts downwards through the Center of Gravity (Center of Mass) of the object.

Why does the buoyant force act?

The reason there's a buoyant force is because of the rather unavoidable fact that the bottom (i.e. more submerged part) of an object is always deeper in a fluid than the top of the object. This means the upward force from water has to be greater than the downward force from water.