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:
- https://www.carls-sims-4-guide.com/tutorials/mods-cc.php
- http://simswiki.info/wiki.php?title=Game_Help:Installing_Sims_4_Package_Files
- https://sims-online.com/install-mods-custom-content-sims-4-tutorial/
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:
General:
Great mod creators: