I am creating a website with React and I couldn't find an answer anywhere. Do I need to write special css codes to make my website more mobile friendly? I tried using "%" method but that did not seem to work. Or do I need to write seperate .js and .css files just for my sites mobile mode? What would be your advice to me?
By "mobile friendly" website, you must mean responsive website. I would suggest using Bootstrap framework for this modern need: https://react-bootstrap.github.io/
You definitely need to learn about the grid system.
You could use your own CSS, but a framework is likely to regroup a whole set of tools you may need in the future.