What you are missing is that collagen is largely comprised of glycine, which is rarer in other proteins. So by eating collagen you are ingesting relatively more glycine to other amino acids, which the body can then use to produce more collagen more efficiently. Of course this doesn’t mean that each glycine molecule from the collagen that you eat will end up in yours collagen, though most might. This is the mechanism, and the effects are confirmed by double-blind studies.
What you are missing is that collagen is largely comprised of glycine, which is rarer in other proteins. So by eating collagen you are ingesting relatively more glycine to other amino acids, which the body can then use to produce more collagen more efficiently. Of course this doesn’t mean that each glycine molecule from the collagen that you eat will end up in yours collagen, though most might. This is the mechanism, and the effects are confirmed by double-blind studies.