The uppermost layer of the skin resembles a brick wall: already dead cells serve as bricks, and skin lipids together with moisture-loving compounds "glue" them together like mortar.
This “brick wall” is called stratum corneum and it is responsible for the skin’s barrier function. In other words, its purpose is not to let things into the deeper, living layers of the skin and the rest of our body. This also means that the products we apply to the skin are most likely to stay on the surface never making it through the “brick wall”. Some products can penetrate deeper though. In most cases, the penetration happens through the “mortar”: the lipids between the dead cell “bricks”. The route is long: our skin’s “brick wall” is 10-15 cell layers thick, and the larger the bricks are, the longer way the compounds need to travel around them through the lipid “mortar”. This route is also only accessible to lipophilic molecules (like retinol) and not water-soluble ones (like Vitamin C). Water soluble molecules can still fight their way through - but they need to penetrate the “bricks” themselves. Ingredients can penetrate deeper into the skin through hair follicles and pores: they can offer a "short cut" for reaching skin's dermis.
Photo by Deepal Pallav Suthar