It goes to the sewer, where it is combined with all the wastewater from all the surrounding houses. And it moves through a set of pipes, until it gets to the wastewater treatment plant.
At the wastewater treatment plant (WWTP), a big screen is used to pull out all the condoms and rocks and stuff. Then it is treated. Solids (like poop) are made to settle out. Bacteria is added to eat up any floating particles of food or poop or other waste. Some chemicals may be added to make the water more or less acidic.
Then, once the water is clean enough (based on government water standards), it is discharged into a local lake or river.
Then it runs around in the world, evaporating into the air, falling out as rain or snow, and so forth. Some of the rain or snow is collected in reservoirs or lakes. Large in-take pipes bring this water into a water treatment plant that makes sure there is no fish in the water and adds fluoride and basically cleans it up a bit before it goes into the water supply and comes out of your faucet.
There is a LOT more detail to it, but that's the general idea.