I wrote a this insight On all objections to collagen.

A couple of points here: one of the most popular objections is “the molecule is not absorbed whole.” These people believe that you supplement collagen by thinking that the collagen molecule (bovine, porcine, or marine) is absorbed whole from the gut and goes whole into the skin to plump it up. This is nonsense and shows the ignorance of these people on the subject.
1) Any protein to be absorbed must be digested, that is, “cut” into smaller pieces that can be absorbed by the intestines. No company selling collagen claims that it is absorbed whole, partly because they all sell collagen hydrolyzed.
2) All collagen is hydrolyzed: this means that the original molecule is “cut” into smaller molecules, the peptides. In this way it is as if it were pre-digested and thus is more easily and quickly digested. So the collagen was not whole to begin with either.

The reason I supplement collagen is glycine. There are several evidences that glycine is a conditionally essential amino acid: our body cannot produce enough of it for daily needs. This happens especially in certain physical conditions: people with metabolic disorders (obesity, type 2 diabetes, fatty liver) have low levels of glycine. These low glycine levels are also often found in cases of inflammation, insulin resistance, high oxidative stress, advanced pregnancy, and exposure to xenobiotics (substances foreign to an organism's normal metabolism, whether of natural or synthetic origin).

Taking collagen (the absolute richest food in glycine) provides our body with this very important amino acid. Glycine accounts for 33% of the amino acids of which collagen is composed and is necessary for the production of glutathione (one of the most powerful antioxidants produced by our bodies) and many other compounds: heme group, purines, creatine, and bile salts.

Important: You don't have to take collagen, just incorporate foods that contain it into your daily diet: gelatin, nerves, joints, or animal skin. Bone broth, contains interesting levels of collagen only if it becomes gelatinous when it cools.

The absolute richest natural food (which has not undergone processing) in collagen is pig skin.