When you cut out the skin panels, they have a curve that follows the curve of that section of hull. The next piece will have its curved edge cut as a mirror image.
A series of small holes is drilled every inch or so along both edges.
The edges are brought together and copper wire is used to stitch the matching pairs of holes together. This results in a curved 'corner' made from two flat panels with no internal framing required.
The glue is epoxy with various fillers added to suit. It's thickened with silica or wood flour and used as a structural adhesive. It can be thickened with microscopic glass 'bubbles' to use as an easily-sanded fairing putty. Straight resin is used with tape or cloth to put a fiberglass sheathing on the plywood.
Because the designer does all the math and develops the lines specifically for flat (ply) panels, it's about the easiest method of building there is. Curves are easy to lay out and cut, epoxy is extremely tolerant of gaps and imperfections in the cuts, and the boat becomes '3D' very early in the build.
That last one is what discourages many people who are building plank-on-frame boats and see flat frames and planks for such a long time.
Even if you haven't built a boat before but know which end of a hammer to hold onto, you can build a S&G boat. If you start with a dinghy, skiff, or rowboat, you'll develop the technique before building the 'big boat'. Then you can use the little boat as a tender.
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