The essence of sewing is simple. The needle draws a thread up through both pieces of fabric, then down, then up so that if you should pull the pieces of fabric apart while the thread is loose there is a net of threads between them. But once the thread is pulled tight and knotted, if you do it right, there is no sign of stitches.
Instead the pieces of fabric merge and suddenly the shape of the whole has changed. What was two dimensional has now become three dimensional all through the work of needle and thread, eye and hand. Magic.