Does RPM refer to the speed of the engine or the speed of the propeller? I followed discussion here: In a constant speed propeller, why does the RPM drop with a prop pitch increase?, I got confused about the RPM there. As the opening statement said in that discussion, "When a constant speed propeller changes its pitch from low to high, RPM would decrease."

From that discussion said that the RPM there is not the RPM of the engine that used to power the airplane, but the RPM of the propeller itself, how fast it rotating after the pitch was changed. The more pitch of course will produce more drag and as the product, the propeller will rotate slower than previously.

Please you masters of the aviation, please confirm if it is true, the RPM is the rotation of the propeller itself and not the RPM of the engine/turbine used to power of the airplane. That have confused me for quite long time and made me very difficult to understand many explanation.
Yes it is the propeller RPM.  If the engine is direct drive, the engine and propeller RPM are the same.  If the engine has a propeller reduction gearbox, then engine RPM and propeller RPM are different, the propeller RPM being the engine RPM minus the gear reduction.

Propeller RPM is controlled by a flyweight governor, like those spinning ball speed regulators you see on old steam engines.  The flyweights control the oil flow to the prop to vary the pitch. The prop control lever applies a spring preload to the governor flyweights and the amount of preload determines at what RPM there is a state of hydraulic equilibrium so there is no pitch change happening. The speed of the governor is a function of the speed of the propeller in either a direct drive or geared engine.