Few weeks ago, there was a Boeing 747 belong to an airliner retired in our country. I am not sure what series is that, but seems it is 747-200. What then will happen immediately to the power plants after it decided to be retired? Is the power plants will be reused, at least for another purpose?
They'll go on another airplane, or go into inventory as spares, or maybe go for overhaul if they are becoming or close to becoming worn out (the can be run until the Inter Turbine Temperature at takeoff or oil consumption becomes excessive, or they fail an internal inspection, or suffer excessive vibration etc). Engines are changed all the time, every few years, and can be thought of as a separate asset from the individual aircraft.