Gravity. It's the defining principle of physics on earth. It affects nearly everything in the universe and is precisely 9.80665 m/s2 at sea level. It also affects how your waste gets from your toilet to the wastewater treatment plant.
Sewer systems are basically wonders of modern infrastructure. Back in the olden days, cities had open sewers where waste flowed through open canals on city streets. One can only imagine the horror...
Today, we're much more civilized: we transport our waste underground in large pipes.
Sewer systems are essential to modern plumbing. Every time you flush a toilet, use a sink or take a shower, you create a liquid waste of varying chemical makeups that have to be transported and properly treated. You could just flush it into a septic tank, but these tanks require maintenance and come with a host of other issues when integrated en masse for modern cities. We have to manage our waste because, well... it stinks. It also contains deadly bacteria and other dangerous chemicals that could affect the environment.
You probably know that wastewater gets treated at a wastewater treatment plant, but let's take a look at how it gets there: sewer systems.
Urban wastewater systems
Urban wastewater systems are needed in densely populated areas so that you don't have to deal with your neighbors... waste. In ideal environments, sewer systems are completely gravity fed, meaning that the pipes slope downward from the source (your toilet) to the wastewater treatment plant. This is done because wastewater has a lot of solids in it, which makes it rather hard to pump. Wastewater also has a lot of bacteria and chemicals, and when we push it through turbulent environments like pumps, it can create dangerous and deadly gasses like hydrogen sulfide.
In an ideal sewer system, pipes from each house or building flow into a sewer main that usually runs alongside a road or underneath it. The sewer main in a large area is usually 3 to 5 feet in diameter, with the pipes from each house being about 6 to 12 inches in diameter on average.
Sewer main infrastructure
In every sewer main, there are periodical vertical pipes that meet the ground with manholes. These access points are spaced depending on the local code and needed just in case a problem arises in the sewer main. Say your toddler has a nasty habit of flushing toys. If they do this enough and the toys make it into the main, then there might eventually be a major clog that city workers need to remove.
Sewer mains will flow into progressively larger pipes as they accumulate more and more sewage, until eventually, they reach a wastewater treatment plant. Usually, these plants are in low-lying areas to make engineers jobs of designing sewer systems much easier.
However, all of this gravity-fed sewer system talk is the ideal scenario. What happens if your house is below the elevation of the wastewater treatment plant or if it needs to travel over a hill? The answer is obvious: you need to pressurize it.
This is done using grinder pumps or lift stations to pump the sewage over hills. These stations collect sewage from lower elevations and utilize pumps to push it up to the needed height where it can gravity flow from there on out. These pumps are specially designed to handle the high amount of solids like toilet paper and flushable wipes.
As a quick side-note: flushable wipes really aren't that. Toilet paper disintegrates in sewer systems, but flushable wipes stay in-tact. They clog up sewer systems and are a pain of treatment plant operators to remove. The reason they're so common is that capitalism won out in the wipes industry compared to city regulation. Wipe with what you want, but keep in mind that somewhere, there's a treatment plant operator that is slightly perturbed with your bathroom habits.
Back to lift-stations. These pumping locations are usually in places of relatively low elevation due to their nature and will probably have a small building to house the equipment. They're also usually placed far away from human population as they have a nasty habit of exploding or, at the very least, producing dangerous levels of hydrogen sulfide due to that turbulent sewage.
Sewer systems, at their core, are a bunch of sloped piped drains that take your waste from your house to a treatment plant. Occasionally there are pumps, but for the most part, gravity does all the work.