How do I change `src` folder to something else in create-react-app

You can use react-app-rewired to override react paths configuration.
In my case, I can change the paths in config-overrides.js file

const path = require('path');

module.exports = {
    paths: function (paths, env) {        
        paths.appIndexJs = path.resolve(__dirname, 'mysrc/client.js');
        paths.appSrc = path.resolve(__dirname, 'mysrc');
        return paths;
    },
}