Collagen injections? How do they work?

Not enough information on Google so yah haha...

I am not familiar with the cosmetic and surgery world BUT, the diet and fitness world is expanding AND so are collagen injections becoming more common.

From what I know, the most common places they are used for are the face and loose skin like lower belly fat and love handles, collage injections tightens up these places. How do they work?
Collagen injections? How do they work?
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