It doesn't matter whether your drinking water is drawn from a well or the mains water supply. The water still travels through pipes of different sizes before reaching your tap. The transition from water mains, which can be as much as 25 cm (10 inches) in diameter or more, to the pipes supplying your tap, which typically are 1.2 cm (1/2 inch), is done in stages. Looking at each stage of your plumbing, you'll see that a plumber connected pipes of different sizes with a reducing fitting. Reducing fittings vary according to the materials from which the pipes they are connecting are made.
Connect PVC water pipes of different sizes with a reducing bushing. It fits inside a coupler glued to the larger pipe and has a hole into which you can insert the smaller one. Choose one that has an outside diameter equal to the diameter of the larger pipe and an inside diameter equal to the diameter of the smaller one.
Glue a coupler to the larger pipe using PVC cement. Spread cement on the outside of the bushing and the inside of the coupler and slide it all the way into the coupler. Spread glue on the inside of the opening and the outside of the smaller pipe, then slide the smaller pipe into the opening.
Solder a copper bell reducer onto a pipe and connect it with one of a different diameter by soldering the other pipe to the other end of the reducer. Bell reducers are, as the name implies, shaped like a bell and fit on the outsides of pipes. Like reducing bushings, they are available in various sizes.
Use a galvanised bell reducer to connect galvanised steel water pipes of different sizes. Screw one end of the reducer to one of the pipes by holding the pipe with a pipe spanner and screwing on the fitting with another pipe spanner. Connect the other pipe by screwing it onto the other end of the reducer in the same way.
Connect a galvanised pipe to a copper one of a different size by screwing a galvanised reducer onto the galvanised pipe and screwing a galvanised nipple, which is a short length of pipe, to the other end. The nipple diameter should be the same as the diameter of the copper pipe you are connecting. Screw a dielectric union onto the other end of the nipple. Solder the copper pipe to the other half of the union and screw the union together with a pipe spanner.
Glue a PVC threaded reducing bushing to a plastic pipe to connect it with a copper pipe with a smaller diameter. Solder a threaded male adaptor onto the end of the copper pipe. Wrap plumbing tape around the threads and screw it into the bushing.
Reducing tees are available to branch a smaller diameter pipe off a larger one.
Always use a dielectric union when connecting copper to steel pipes. It prevents the pipes from touching and creating an electrical charge that will corrode the pipes.