The type of 3D printer I have seen in action produces a solid block of plastic, layer by layer. It works like an ink jet printer. Two "inks" are used. One ink is soluble in a solvent, the other isn't. When you are done printing the block, paper-thin layer by paper-thin layer, embedded in a block of soluble plastic is a complete device in a more substantial plastic.
You put the solid block in a solvent and what is left is a complete working "assembly" of parts, which don't have to be assembled. The complexity can be greater than a normally manufactured design, or simpler, depending on your perspective. The point is it comes out ready to use.
