What is the best combination of technologies for front-end with React in order to become a full-stack developer?

Question

Backend

Hello, I know the basics for the frontend html, css, js, react, etc. What is the best thing to learn in such a bundle for the backend? If I learn node js will it be in demand for freelancing? Or all the same php, help me figure it out. It's not about complexity, it's about demand. I can also add to this that for the time being I live far from offices and the world of it. Therefore, I focus on the demand for freelancing