Yes, a lot of them really do help to firm up the skin – all over the body – and plump it up by improving skin hydration, too. What you need to look for is one that gives you 7,000-10,000mg of collagen per daily dose, and that collagen needs to be hydrolysed (ie to have had its large molecules smashed into tiny fragments that can be absorbed in the blood). If you can discover the size of the product’s molecules, the smaller the better. If that molecular size is 2 or 3 kda (kilodaltons), that’s terrific.
The brand I’ve chosen for my shop is Totally Derma, which is expensive, but it is particularly good at building collagen and hydration in the skin. This is because it has two patented-and-proven ingredients, one for regenerating collagen, joints and tendons, and another for improving hydration, so together they give particularly good results.