(Intended for education, not medical advice.)
Let's start with some basic skin vocabulary we'll need. (I already see the eyes rolling. I'll keep it painless.). Imagine your skin as a cake. The frosting is called the stratum corneum, too much of which can cause scaly skin. The next layer is the epidermis, which is where many skin cancers can form. Ultraviolet light can cause many changes in this layer, including sun (age) spots. The dermis comes next, and is largely formed by collagen. This is the layer most fillers are placed to enhance the volume of the skin. Lastly, is the subcutis, or fatty layer. Hair follicles can be found here. One of the things to remember is that topical skincare products generally have an easier time getting to the upper layers of skin (stratum corneum, epidermis) than the lower layers. For example, putting hyaluronic acid on the surface of the skin can help with moisturizing and hydrating the upper layers, but you'll need to inject it (a different form of it) into the dermis to get the volumizing effect of fillers. I'll mention these skin layers to give us a sense of what and where things are happening with skincare products.
Kay K. Sung, M.D.