The heater core is the 2 rubber hoses going into the car.
I believe he used bulk 5/8" heater hose to connect to the heater core.
Line in for water and chemical and a return line to a bucket.
Regarding CLR it says not to use on aluminum.
Plenty of people do use it on heater cores.
I would try not using too much. Flush out really well.
I have never done| this. Just sharing what I know .