Skin details are the CAS layer most people skip and then wonder why their sims look slightly uncanny. Vanilla skin is flat. No freckles, no moles, no cheek blush that actually sits on the face, no body hair that looks like anything other than a shadow. Community skin overlays fix all of this with a few CAS clicks.
We've pulled together about 275 skin detail picks: full skin overlays that replace the base texture, freckle maps, mole placement sets, nose bridge details, cheek blushes, body hair, pregnancy stretch marks, and the specialty stuff like aging details and sunspots. Pralinesims and Obscurus-Sims turn up a lot here because they've been shipping realistic skin overlays for years, but there's a long tail of smaller skin creators doing great work on specific features.
Most of these stack — you can layer a skin overlay, freckles, a blush, and a nose detail all on one sim. The trick is stopping before you over-CAS them into looking like a different art style than the hair and clothes you're using.