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 beauty store, you'll see creams, serums, fluids, and toners with pictures and descriptions of how they'll make you look younger 
. Unfortunately, this is all a lie. The argument is very simple: this acid is needed inside the tissues, not on the skin.
And its synthesis depends on various nutrients and your metabolism. For more details, see the links at the end of the post; there's an article in Russian by a biochemist.
Sunscreens. They're necessary, but not always and not everywhere. For example, in the climate of my city, Kyiv, it's more of a marketing ploy. But somewhere closer to the equator, they're a necessary thing. For example, when I was in Indonesia, I was surprised by the locals walking around in jackets in 35°C (95°F) temperatures. Later, when I got sunburned, it became clear why . Standard SPF 50+ creams aren't enough there either; for example, surfers used a special clay-like paste with zinc oxide that wouldn't wash off all day. It's also important to understand that extremes are bad—without sun, we don't get vitamin D and don't train our antioxidant system. But too much sun also causes mutations in skin cells. Keep this in mind and avoid tanning beds.
Cosmetics for problem skin . These products typically contain acids: salicylic acid, azelaic acid, and others. Applying them to the skin kills both bacteria and the skin itself. This 
causes peeling, which is the dead skin cells of the basal layer. New cells then appear. This process is portrayed as beneficial and effective. In reality, it's deliberate damage to already problematic skin in anticipation of regeneration. This may clear up acne for a while, but it won't cure it. And you'll be told it's a chronic condition and you need to apply it constantly.
Shampoos and cleansers. The primary purpose of these products is to wash away dirt. Cheap products do this aggressively, while expensive ones do it gently. But don't expect them to improve your skin or hair. For example, shampoo manufacturers add special silicones to make hair shiny and smooth. Cleansers contain substances that mask pores. But all these tricks provide only temporary results.
Now a few words about cosmetics that are actually beneficial. These are mostly Asian manufacturers (Japan, South Korea). I won't advertise anything, but I will point out some things to consider:
Cosmetics with glutathione , cosmetics with coenzyme Q10 (but not all forms of this substance work; you need to try different manufacturers), cosmetics with fucoidan (I think only Japan makes them), peptide cosmetics can help with fine wrinkles and improve skin tone (for example, copper tripeptide), and cosmetics with plant extracts (for example, centella asiatica, pomegranate extract, etc., there are many) can reduce inflammation. If you have problem skin, avoid creams and any oil-based products. Only gels, serums, and toners. Cosmetics with probiotics can help with dermatitis and acne by altering the skin's local immunity. Shampoo with aminexil can accelerate hair growth.
And finally, remember that the condition of your skin, hair, and nails depends primarily on your genes, secondarily on your current metabolism, and only lastly on your skincare routine.
For example, I'm sure you all know someone with perfect skin who never uses sunscreen or any other creams. Think about that.
https://habr.com/ru/post/569954/
http://www.ncbi.nlm.nih.gov/pmc/articles/pmc8627341/
https://pubmed.ncbi.nlm.nih.gov/35083444
https://pubmed.ncbi.nlm.nih.gov/33526970
https://pubmed.ncbi.nlm.nih.gov/26648450
https://pubmed.ncbi.nlm.nih.gov/26848220
