Here’s an “ingelious” way to understand upthrust in JC A Level H2 Physics Forces based on Principle of Floatation.
When an object is submerged in the fluid, it will sink and displace the fluid.
You could think of the molecules of the fluid trying to reject the “foreign” object from infringing into their space and tries to perform a pushback.
This pushback can be seen as upthrust.
When mg > U, the object continues to sink and displaces more fluid.
This in turns increases the upthrust, and the object will continue to sink until it balances against the weight of the object.
However, if the upthrust fails to balance the object’s weight depsite being fully submerged, it will reach to the bottom of the container until it is supported by the additional normal contact force from the base of the container.