What does a plastic welder do? What are their specific jobs?

I know there are two kinds of plastic welders. One is to weld small-diameter pipes, just like water pipes at home. Plastic welding torch welding is used, which is similar to blowing hot air with a blower to heat and melt plastic-covered electrodes and weldments. In addition, those large-diameter black HDPE pipes are heated with special welding tools and then extruded and bonded.