Skincare and cosmetics are different categories with different success metrics. Skincare aims to change skin function over months. Cosmetics aim to improve appearance for hours. Many products marketed as skincare are functionally cosmetics — useful, but you should understand which is which.
What's actually skincare
Retinoids. Sunscreen. Acid exfoliants. Vitamin C, niacinamide, peptides. These have documented effects on skin function — collagen production, barrier repair, pigment regulation. Effects accumulate over weeks to months.
What's actually cosmetic (but marketed as skincare)
Most primers and 'glow drops'. Light-reflecting moisturisers. Most 'instantly plumping' serums. Most face oils (some genuine barrier effect, but mostly aesthetic). These improve the surface for hours; nothing changes underneath.
Use both — they serve different purposes. Just don't expect a primer to do what tretinoin does, or a glow drop to replace vitamin C.