[gamblor956]

Digestion doesn't work like that. A great deal of food is not fully broken down by digestion. In fact, many functions of the human body rely on various nutrients/whatever surviving their passage through the gut.

Most, but not all collagen is broken down into collagen peptides in the gut. Most of those collagen peptides are then broken down into amino acids. But not all collagen peptides are broken down, and the peptides that remain can enter the bloodstream where they can be utilized as needed.

(The collagen that doesn't get broken down in the gut tends to come out the other side of the digestive tract intact, because unlike collagen peptides, collagen is too large to enter into the bloodstream and there are no organs after the intestines that can make direct use of collagen.)