Which trade is responsible for assembling and aligning underground pipes outside a structure?

The trade responsible for assembling and aligning underground pipes outside a structure is indeed the Pipe Layer. Pipe Layers specialize in the installation and maintenance of pipelines that transport water, sewage, and other fluids. Their work involves digging trenches, laying pipes, ensuring proper alignment, and connecting the pipes to existing infrastructure. This requires a good understanding of grading, drainage, and soil conditions to ensure that the pipes are set correctly and can function effectively.

In contrast, Millwrights primarily focus on the installation, maintenance, and repair of machinery and industrial equipment within a facility, which is not related to underground piping. Pile Drivers specialize in the installation of piles for foundations and structures, involving different skills and equipment from those used in pipe laying. Sprinkler Fitters are experts in the installation of fire protection systems, primarily working indoors and on above-ground systems rather than on underground piping systems. Thus, the expertise and responsibilities of each trade clearly delineate the Pipe Layer as the correct choice for this question.