Those are MLPs that own and use the pipelines. Do they manufacture the pipe and construct the line also or do they contract other firms for that?
The operating companies basically only operate. They hire companies like the ones I have worked for to engineer and construct their pipelines and associated facilities.
They or their contractors will then purchase from manufactures the equipment and material to build the pipeline and associated facilities.
They do not even own the oil or gas moving in them. They really operate like a trucking company or railroad. Someone else hires the pipeline company to move their product.