In creams, collagen is a moisturizer. The molecule is far too large to enter skin, so it cannot replace or rebuild your own collagen.
Collagen is the main structural protein of the dermis. In skincare it is usually animal- or fish-derived collagen or smaller hydrolyzed fragments. An intact collagen molecule weighs around 300,000 daltons, while molecules much above 500 daltons don't get through the skin's outer layer, so collagen stays on the surface.
There it works as a humectant and film-former: it holds water and leaves skin feeling smoother and plumper for a while. That is real but temporary, and it's the same job glycerin or hyaluronic acid does. 'Collagen' serums that claim to boost your own collagen rely on other actives, such as retinoids, vitamin C or peptides, for any effect. Oral collagen supplements are a separate question with their own evidence.
No credible evidence shows that collagen applied to skin increases skin collagen or firmness; the size limit on skin penetration is basic physiology. Its moisturizing effect is real but modest and short-lived.
Argues that a compound must weigh under about 500 daltons to pass through the skin's outer layer, based on the size of known contact allergens and of the drugs that work topically or through transdermal patches.