There ya go
Water normally flows in one direction around engine and to radiator.
Hot water into top of radiator from engine
Cold water from bottom of radiator to engine.
So to flush radiator, where gunk is most likely, hose into bottom of rad and flush so jammed gunk is pushed out, not further in.
But messy unless you can get a good seal on bottom pipe connection or remove radiator.
Engine flush easier as you can stuff a hose pipe into top hose and back flush it that way.
With a newish engine and no worries that coolant has broken down (looks sludgy), I simply refill with water, run it a little to circulate, then drain down again.
If more concerned, I've taken to following advice on here and popping a dishwasher tablet (without powerball) into the system and let it work for a day before draining and flushing.
Heater setting.... good practice as not all cars work on flaps.