where do you think the coolant goes after it leaves the core, restricted? If part of the system is restricted wouldn't the entire system be slowed down?
No On a lot of cars when you shut off the heat you completely shut off the flow to the heater core.
a heater core is more like a by pass. Your main flow are the radiator hoses
For the heater core usally the intake side is picked up at the top of the engine (last place the water flows)
And the discharge goes into the upper part of the radiator
When your not using the heat the heater core is doing nothing to help cool the engine
That's why if you over heat in the summer it helps to run the heater full blast it removes some of the heat
from the water