When you empty a bath, basin or sink, or flush a WC, the waste water heads for the drains. In a modern home, each water-connected appliance - including the WC - has a waste pipe that runs directly into a single vertical soil stack, normally sited inside the building. In older properties, sinks, basins, and baths empty into ground-level gullies or wall-mounted hoppers, while WC waste is discharged into a separate external soil pipe, and the two parts of the system do not merge until they get underground.
The various waste and soil pipes are connected into underground drainpipes that merge in below-ground inspection chambers with metal covers. The drainpipes then run on, via other connecting chambers if necessary, towards the nearest sewer.
Rainwater from gutters runs via separate downpipes to ground level, and then via underground pipes to the surface water drain under the nearest road, or to drainage pits called soakaways if no drain is nearby.
