Collagen is a fat protein, a structural component of our skin, bones, tendons, blood vessels, cartilage. It doesn’t dissolve in blood. It is not absorbed across membranes or blood vessel walls.
So if it were injected into our blood it would probably cause clots and strokes.
Ingesting collagen however, breaks down collagen into smaller amino acids, which are then reabsorbed into blood, taken up by the cells as resource, and then used by the skin!
It all comes down to bioavailability— why the SIZE of collagen matters (so smaller the better). Hence why Crushed Tonic only uses marine collagen. In addition to being hydrolyzed (aka water to sterilize and make the cells even more digestible).
Collagen fillers are put directly into the skin (like fat cushions into the skin tissues), so they forcibly change the appearance in a different way!