Nissan Patrol 4x4 (Africa)

Hello!
I funny question came into my mind: how does the water get from the expansion tank back into the cooling system?

Obviously if the radiator overflows the pipe at the top lets the water flow into the expansion tank.

But how does it go back? Is it via ventury effect on the same way?

Sorry for the stupid question, but I just don't get it.

The radiator cap has a small valve in the middle. This may look like a little rubber "button", or even just like a rivet.
As the water cools down, the volume in the radiator reduces. This causes a vacuum, which in turn opens this little valve, and the water is sucked back in.

Thanks, Peter!