Using mods and CC (which stands for custom content) in Sims 4 is actually pretty easy and there are many tutorials on how to do it:
One more advanced thing is using an Overrides folder to resolve some mod conflicts and makes sure everything runs smoothly.
How to setup mods Override https://www.patreon.com/posts/37922404
Some websites to get you started with mods and cc:
Great mod creators: