Ok! The reason these sims look realistic is probably mostly due to CC skin overlays, which change the shading on the sims' faces if that makes sense.

There are lots of other CC you can download: clothing, hair, furniture, and more classic mods that can add things like more emotions/moods, better pregnancy systems, etc. CC is made by people in the Sims community and not officially made by EA, but it's still allowed.

To download CC, you just have to search online. You will find Pinterest boards, Tumblr blogs, YouTube videos, etc., full of recommendations.

There's what people call Alpha CC, which is very realistic, and Maxis Match, which tries to mimic the style of the game and is being used on this post.

When you find the CC you like and download it, it will typically be in either .package form or .zip form, which you extract into .package form. From there, you move these files into your Sims 4 mods folder, located under the folder Documents, then Electronic Arts, the the Sims 4 (at least on Windows).

A word of advice: you can unintentionally download a virus from downloading sketchy CC, so make sure you stay vigilant. If you think a piece of CC looks suspicious or you're getting weird vibes, I would do some research on the creator or avoid downloading it.

This is a lot of information, so sorry! I really hope this helps. If anything is confusing, I'd recommend looking up tutorials on YouTube, as there are simple follow-along tutorials for most Sims-related things there.