You don't want an explanation of how plumbing works...but your magical folks don't seem to have much alternative available but to build on the basic physical principles....
Water and sewer pipes can be made of just about any material. Pipes have been made of masonry, wood, metal, plastic, and many other materials.
Sewer pipes operate on the principle of simple gravity - water flows downhill. Since sewers also contain solids and semi-solids, the downhill slope (called pitch) of the pipe is important. Best pitch is 1/4-inch for every foot of pipe. Sewer pipes must be placed several feet below water pipes, so leaky sewers don't pollute the clean water supply. Sewer diameters must be large enough for cleaning.
Most sewers today empty into a waste treatment plant, because the next town downstream is eventually going to drink it. It typically takes 3-5 days for microorganisms to consume most waste in an open waterway like a river, and for healthy oxygen and ph to resume. If the town downstream is closer than that, they may get sick frequently or seasonally.
Water pipes today operate on the principle of pressure. The easiest way to get pressure is a central water-tower and a single central pump lifting water into the tower. The central tower must be sized for maximum demand, which is usually early morning...or else folks run out of water during their morning ablutions. Higher towers can cover a larger area, but are more expensive to build and cost more to pump. Pressure means small, pressure-capable pipes (but not too small), and pitch isn't relevant. The central source also makes drinking-water treatment easier for everybody. Water pipes are typically buried to prevent freezing, prevent contamination, and to pass under streets.
Clean water treatment consists of dosing the water with a coagulant (like lye to get rid of suspended solids) and a disenfectant (ozone or chlorine to get rid of microorganisms), balancing the ph, etc.
Older water systems that were gravity-drawn (aqueducts) usually fed above-ground only to neighborhood fountains or outlets. Crossing streets and feeding into (the basement of) each house was simply too hard, too expensive, and not convenient enough to bother.
...so it seems like your folks can make better pipes and stronger towers. They can make incremental improvements that make their water supply a bit safer, waste a bit less, pollute a bit less. But no fundamental changes.