Hi again Mozart
That is due to a patented fan design.
More info here. https://www.cultofmac.com/267222/how-the-imac-cooling-fan-stays-so-silent/
And then they have pretty large heatsinks on their CPU’s and RAM.
But again, it’s all about what kind of fan you are using. My workstation is a Dell T3500 with an X5687 processor. The processor is passively cooled by two fans on the chassis. And they are dead silent.