Speed tour of how plumbing works...
Here's how plumbing works.
Inbound - water comes into your home from the city supply or your well (depending on where you live). Once in the house, it splits off where some pipes go to your hot water heater (or heaters) and the other brings cooler water into your house. The pipes are usually made of copper or in newer homes may be PEX. Because the water is under pressure coming into the house when you open the tap, the water flows.
Outbound - water flows out of your fixtures (shower, sink, toilet, washing machine, etc.) into a series of PVC pipes that are in your house. Those pipes are connected into vertical stacks that go to the basement of your house and then flow either into the sewer or your drain field. To balance air, and make sure the water(ish) flows down out of your house you have pipes that poke through your roof. These are called vents.
Water is lazy and always flows to the path of least resistance. If it starts to "pool" that's when trouble starts.
Have a great day!