In short, the effectiveness of most cosmetics is illusory and inspired by marketing, and we can stop there, but I will still provide minimal scientific arguments
. I will do this for popular categories of cosmetics.
Products with hyaluronic acid . In any cosmetics store you will see creams, serums, fluids, tonics on which it is drawn and written how you will look younger after using them 
. Unfortunately, this is all a lie. The argument here is very simple - this acid is needed inside the tissues, not on the skin.
And its synthesis depends on various nutrients and your metabolism. If you need details - see the links at the end of the post, there is an article in Russian from one biochemist.
Sunscreens. They are needed, but not always and not everywhere. For example, in the climate conditions of my Kiev, this is more marketing. But somewhere closer to the equator it is a necessary thing. For example, when I was in Indonesia, I was surprised by the local savages who walked in jackets on the street at +35. Later, when I got sunburned, it became clear to me why. But the standard SPF 50+ cream is also not enough there, for example, surfers used a special clay-like paste with zinc oxide that did not wash off all day. You should also understand that extremes are bad - without the sun, we do not get vitamin D and do not train the antioxidant system. But when there is a lot of it, mutations occur in skin cells. Remember this and do not go to the solarium
Cosmetics for problem skin . These products usually contain acids: salicylic acid, azelaic acid and others. A person smears them and poisons both the bacteria on the skin and the skin itself 
. Peeling occurs, which is dead cells of the basal layer of the skin. And then new cells appear. And this process is passed off as beneficial and effective. In fact, this is a targeted damage to already problematic skin in anticipation of regeneration. Acne may go away from this, for the time of use, but it cannot be cured. And you will be told that this is a chronic disease and you need to smear it constantly
Shampoos, cleansers. The direct purpose of these products is to wash away dirt. Cheap products do it aggressively, expensive ones do it delicately. But you shouldn't think that your skin or hair will be better after them. For example, shampoo manufacturers add special silicones for shine and smoothness of hair. They add substances to cleansers that mask pores. But all these tricks give a very temporary effect.
Now a few words about cosmetics that are still useful. These are mainly Asian manufacturers (Japan, South Korea). I will not advertise anything, but I will say what you can pay attention to:
Cosmetics with glutathione Cosmetics with coenzyme Q10 (but not everywhere the working form of this substance, you need to try different manufacturers) Cosmetics with fucoidan (I think only Japan makes it) Peptide cosmetics can help with fine wrinkles and improve skin tone (for example, copper tripeptide) Cosmetics with plant extracts (for example, centella asiatica, pomegranate extract, etc., there are many of them) can relieve inflammation. If you have problem skin, avoid creams and any oil base. Only gels, serums, tonics. Cosmetics with probiotics can help with dermatitis and acne by changing the local immunity of the skin. Shampoo with aminexil can speed up hair growth.
And finally, remember that the condition of your: skin, hair, nails - first of all depends on genes, secondly on current metabolism, and only lastly on care.
For example, I think each of you has friends with perfect skin who have never used sunscreen or any other creams. Think about it.
https://habr.com/ru/post/569954/
http://www.ncbi.nlm.nih.gov/pmc/articles/pmc8627341/
https://pubmed.ncbi.nlm.nih.gov/35083444
https http://pubmed.ncbi.nlm.nih.gov/33526970
https://pubmed.ncbi.nlm.nih.gov/26648450
https://pubmed.ncbi.nlm.nih.gov/26848220
