A single car has about 30,000 parts, counting every part down to the smallest screws. Some of these parts are made at Toyota, but we also have lots of suppliers that make many of these parts. The 30,000 or so parts use different raw materials and different manufacturing processes. Making good parts requires a lot of research and development. Having the parts made by companies that have expertise in their own fields allows us to obtain high-quality parts. Toyota's suppliers make seats, wheels, steering wheels, windshields, headlights, and meters, for example.
Toyota uses a system in which the necessary parts are made in the needed amounts at the right times. A card called a "kanban" is attached to each parts box and describes the types and quantity of parts needed. This card also prevents the parts factories from making too many parts and the assembly process from having to allocate space to store parts that are not immediately needed.
As just mentioned, about 30,000 parts are needed to make a car. Since Toyota makes cars exactly the way customers order them, each car is unique. That means that different parts have to be fitted in different cars. Using different parts results in different prices for the individual cars.