Loading React into WP
Create a child theme, or if you want to load React into a plugin, you can do this in a plugin.
Make sure you have node and npm installed globally in your machine.
Make sure you have some way to access the command line.
npm init in the child theme to enable it to use npm packages. You now see a package.json file in your child theme directory. Listed here are the libraries your application depends on. It's a bit like wp_enqueue_scripts, but for JS libraries. All the external libraries/scripts you need are listed under devDependencies or dependencies.