Rebuild is where a mechanic swaps out car parts that are worn out n replaces then with new spare parts.Overhaul replaces engine parts like piston rings,head gasket,engine bearings,top cover seals and valves to bring the car back to life.
Replacement of then engine on the other hand is where you buy a complete engine that was used in another car.There are many pros and cons for example a rebuild can maintain a cars original value and longevity,on the other hand replacement diminishes a cars value.Ultimetely the decision should be based on balance of cost,time,vehicle condition and impact on value.
Always use genuine auto parts while rebuilding for longer and better service