Skin doesn’t just absorb things. In fact it’s incredibly good at resisting so much so that most beauty products are complete BS.

If you look at the skin as a protective barrier, then that makes sense, but its ability to absorb cosmetics depends on a lot of factors. First off, what makes a product more absorbable (i.e. influence percutaneous absorption)? Molecular size (typically smaller than ~500 Daltons). Lipophilicity (fat-solubility). Formulations and vehicle, e.g. emulsions (oil-in-water, water-in-oil) can improve penetration depending on balance. There are alcohol-based solutions that enhance delivery of certain actives but may irritate. There are other penetration enhancers such as propylene glycol, ethanol, fatty acids, which disrupt the skin barrier slightly to allow deeper diffusion. Heck, even salicylic acid does that.

There are a lot of other factors here, such as your skin's condition (hydrated skin absorbs better), damaged or inflamed skin even more but sometimes to unsafe levels and it is typically contraindicated for almost all cosmetics.

In any case, most cosmetic ingredients act locally (i.e. not supposed to enter into systemic circulation like transdermal drugs), improve hydration, texture, and/or appearance by altering the stratum corneum or slightly beyond. Systemic absorption is limited unless specifically engineered to do so, such as nicotine patches, hormone creams, fentanyl patches, etc. I mentioned this below "transdermal drug delivery".

The curious should look up the differences between cosmetic absorption vs. transdermal drug delivery as well. For example, cosmetics are not intended to penetrate into the bloodstream, hence the surface layer depth. To give you percentages, typically >90% remains on skin surface, but it also depends on what you want to achieve, because for example hyaluronic acid in creams are of large molecule (~3000-5000 Da), meaning it essentially 0% penetrates. It hydrates only by trapping water on the skin surface. Important to note here that sometimes this is exactly what people want, i.e. this surface hydration is what gives the "plump, glowing skin" effect people expect, so if the goal is hydration and surface smoothness, then large HA is ideal (surface action is enough), but if the goal is true wrinkle reduction or anti-aging, then surface HA alone is not sufficient. This is why companies combine HA with retinoids, peptides, or vitamin C, which act deeper and can influence collagen production. If the goal is long-term structural change, then you can have injectables (such as dermal fillers, which are being used for enlarging the lips, for one).

Transdermal Drug Delivery on the other hand are supposed to enter the bloodstream so drugs delivered through skin (e.g., nicotine, fentanyl, estradiol patches) are engineered to bypass the stratum corneum barrier. They use optimized molecular size, solubility, enhancers, and occlusion. If you want percentages here as well, I would say 20-95% systemic absorption of applied dose, but it depends on a couple of factors I have previously mentioned.

Just to stay on topic: sunscreens require only surface layer depth of absorption only, and in fact, many products work at this level. Their effectiveness depends heavily on formulation, proper application, and reapplication. Sunscreens do work when used correctly, they significantly reduce UV damage, premature aging, and skin cancer risk, BUT you must apply it properly and reapply often. You should combine with shade, too.