Niacinamide and hyaluronic acid, what each one is doing
These two turn up on nearly every label, and the labels rarely say what either is for. Here is the short version.
Hyaluronic acid
A humectant. It holds many times its own weight in water, and on the surface of the skin it keeps water where you want it rather than letting it go. That is what people are describing when they say skin looks plumper after using it.
Niacinamide
A form of vitamin B3, used in cosmetics for the look of an even tone and a smoother surface. It is well tolerated by most people, which is why it appears at low concentrations in so many products.
Why they are paired
They do different jobs and neither interferes with the other. Water retention and the look of tone are two separate complaints, and most people have both.
Patch test first
Well tolerated is not the same as tolerated by everybody. Try a small area first, especially if your skin reacts to new products.