VW newcastle bad service I've never seen.

On the 3rd of August 2020 i took my car to Vw Newcastle Registration No HH 00 MD GP for radiator repair the diagnostics my car and found that radiator and air con needs to be repair then I gave them go ahead to fix it then it was fixed they came back to me they the car is still over heating they to check cylinder head after they checked it they said the egen has demaged they hv to remove it and put the new one ,when I brought my my car to repair the egen light was not showing on the dash board they removed my egen and put the second hand one after putting the egen thy said I must buy a water pump was bought and installed after that they came back again and say the car is still overheating the told me the problem is turbo....MY QUESTION IS HOW COULD A CAR IN VW BE INSTALLED A SECONDHAND EGEN AND WHEN THEY DIAGNOSE THE CAR IT MUST SHOW ALL PROBLEMES THAT NEEDS TO BE FIXED NOT ONE BY ONE THATS MY CONCERN AND THE CAR WHEN I BOUGHT IT TO VW REPAIR IT WAS NOT HAVING ENGEN PROBLEM IT WAS JUST LIKING WATER, on the 18th September we went there to see the car we spoke with them and they promised that they will shop around to get Turbo since then we haven't heard from them.