Short answers, pulled from the story.
The three parts are the duodenum, jejunum, and ileum. The duodenum neutralizes stomach acid and begins preparing nutrients for absorption; the jejunum absorbs most nutrients including sugars, amino acids, and fatty acids; and the ileum absorbs vitamin B12, bile salts, and any remaining digestive products.
The length varies widely, ranging from as short as 3 metres to as long as 10.5 metres depending on the individual and the measuring technique. Taller people generally have a longer small intestine, and measurements taken after death tend to be longer than those in living subjects.
The surface area of the human small intestinal mucosa averages 30 square metres. This is achieved through three layers of structure: circular folds in the mucosa, finger-like villi projecting from those folds, and microvilli on the surface of individual epithelial cells.
Peyer's patches are clusters of lymphoid tissue located within the ileum. They sample antigens from potentially harmful bacteria and microorganisms passing through the digestive tract and present them to the immune system, serving as an early-warning component of the body's defenses.
By the fifth week of embryological development, the ileum grows so rapidly it protrudes through the umbilicus as a U-shaped primary intestinal loop. Between weeks six and ten it rotates anticlockwise, and after returning to the abdomen around week ten it rotates a further 180 degrees, which creates the twisted arrangement of the adult gut.
Vitamin B12 can only be absorbed by the ileum after it binds to a protein called intrinsic factor. Without this binding step, the ileum cannot take up the vitamin, making the terminal ileum the exclusive site for B12 absorption.