They plot,cut then glue the mylar panels together then they lay the mylar sail on to the mold then they lay the carbon strands over the mylar while doing this they are heating the sail and the glue so the cloth and strands take the shape of the mold. it's a bit like heat shrink. You can sew the mylar panels together but the glue is stronger than the cloth. These are crosscut sails with glued yarns. I havew a picture of the Maxi yacht Wild Oats 11 in a mag and you can clearly see that it is a crosscut sail with load path yarns. Have a look at the Genoa where the battens are the are seams.