Beginner Tips for a Front-End Developer

Hey. I’m new to coding and I’ve gone through Mosh’s HTML/CSS course. Now, I know that you can use HTML/CSS to make the backbone of a website but how do I add functionality? Do I use JS, or React, or something else?
And then, after this, do I then learn JS/React?

tldr(kinda): What makes a website work? What is JavaScript (pls provide an example) and what is React (pls provide an example) and what do I after I’ve learned those things?

Yes, you should learn Javascript (JS). HTML provides structure. CSS provides styling. JS is a programming language, which can be used in the front end and the backend (see something called Node).

React is a front end framework, built on JS.

You might actually want to tackle Python and Django first, simply because Django is a framework built on Python and doing that may give you a better idea of how backends and frontends work together.