It's done by a process known as "lofting". Originated in ship building
after my apprenticeship at RR i went into the aerodynamic fairings Design office, where i learned to loft engine fairings and doors and pylons etc.
we used to draw on sheets of nice shiny aluminium with lots of french curves and trammels, with erasing done by emery cloth, these lofts were scale 1:1 and were taken up to Hucknall to be carefully cut out to make egg box molds, from which tooling was made to profile aluminium sheets. being rascals we used to put the shiny aluminium off cuts on the floor when the tea trolley came around we played guess what colour
also believe it had something to do with parachute packing on long tables which were in a loft as it was the only space long enough for these tables.
we used to draw in ink on finest Irish linen which could have the starch soaked out to make cracking hankeys and dusters. i still shake my pencil when drawing as a throw back to getting the ink flowing in the old pen knibs.
with regards to the bits between the sections, we used to rely on the toolmakers eye to blend it all in nicely, which CAD does for us now.
The problem is nowadays that people forget that the computer is just a tool of the job, as was the drawing board.
and in some cases the limitations of the tools or the operators can influence the design solution rather than looking for another tool, like the one in your head.
CAD is great though and using virtual reality CAD to see if you can close an engine door latch without lying down is a real step forwards in terms of designed in safety
.