This is not intended to accurately represent scale or true design
In this diagram the water from the roof enters the tank via a filter which is designed to flush leaves and other debris away when there is sufficient rain. Excess water overflows to the surface water drain. Water travels to a pump where it is sent under pressure to the toilet cisterns and washing machine.
The tank in this instance could just as easily be underground in which case a submersible pump would be installed within the tank