How does react-create-app/react-scripts find the entry point?

you can run npm run eject so that the project turn back to a normal webpack project. then you can find the configs.

If you look at package.json, the start script is defined there:

"scripts": {
  "start": "react-scripts start",

It runs react-scripts start. And the package.json file for react-scripts looks like this:

"bin": {
    "react-scripts": "./bin/react-scripts.js"
  },

So we should see what does ./bin/react-scripts.js start do, which runs this code:

// @remove-on-eject-begin
/**
 * Copyright (c) 2015-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
// @remove-on-eject-end
'use strict';

// Do this as the first thing so that any code reading it knows the right env.
process.env.BABEL_ENV = 'development';
process.env.NODE_ENV = 'development';

// Makes the script crash on unhandled rejections instead of silently
// ignoring them. In the future, promise rejections that are not handled will
// terminate the Node.js process with a non-zero exit code.
process.on('unhandledRejection', err => {
  throw err;
});

// Ensure environment variables are read.
require('../config/env');

const fs = require('fs');
const chalk = require('react-dev-utils/chalk');
const webpack = require('webpack');
const WebpackDevServer = require('webpack-dev-server');
const clearConsole = require('react-dev-utils/clearConsole');
const checkRequiredFiles = require('react-dev-utils/checkRequiredFiles');
const {
  choosePort,
  createCompiler,
  prepareProxy,
  prepareUrls,
} = require('react-dev-utils/WebpackDevServerUtils');
const openBrowser = require('react-dev-utils/openBrowser');
const semver = require('semver');
const paths = require('../config/paths');
const configFactory = require('../config/webpack.config');
const createDevServerConfig = require('../config/webpackDevServer.config');
const getClientEnvironment = require('../config/env');
const react = require(require.resolve('react', { paths: [paths.appPath] }));

const env = getClientEnvironment(paths.publicUrlOrPath.slice(0, -1));
const useYarn = fs.existsSync(paths.yarnLockFile);
const isInteractive = process.stdout.isTTY;

// Warn and crash if required files are missing
if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
  process.exit(1);
}

// Tools like Cloud9 rely on this.
const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
const HOST = process.env.HOST || '0.0.0.0';

if (process.env.HOST) {
  console.log(
    chalk.cyan(
      `Attempting to bind to HOST environment variable: ${chalk.yellow(
        chalk.bold(process.env.HOST)
      )}`
    )
  );
  console.log(
    `If this was unintentional, check that you haven't mistakenly set it in your shell.`
  );
  console.log(
    `Learn more here: ${chalk.yellow('https://cra.link/advanced-config')}`
  );
  console.log();
}

// We require that you explicitly set browsers and do not fall back to
// browserslist defaults.
const { checkBrowsers } = require('react-dev-utils/browsersHelper');
checkBrowsers(paths.appPath, isInteractive)
  .then(() => {
    // We attempt to use the default port but if it is busy, we offer the user to
    // run on a different port. `choosePort()` Promise resolves to the next free port.
    return choosePort(HOST, DEFAULT_PORT);
  })
  .then(port => {
    if (port == null) {
      // We have not found a port.
      return;
    }

    const config = configFactory('development');
    const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
    const appName = require(paths.appPackageJson).name;

    const useTypeScript = fs.existsSync(paths.appTsConfig);
    const urls = prepareUrls(
      protocol,
      HOST,
      port,
      paths.publicUrlOrPath.slice(0, -1)
    );
    // Create a webpack compiler that is configured with custom messages.
    const compiler = createCompiler({
      appName,
      config,
      urls,
      useYarn,
      useTypeScript,
      webpack,
    });
    // Load proxy config
    const proxySetting = require(paths.appPackageJson).proxy;
    const proxyConfig = prepareProxy(
      proxySetting,
      paths.appPublic,
      paths.publicUrlOrPath
    );
    // Serve webpack assets generated by the compiler over a web server.
    const serverConfig = {
      ...createDevServerConfig(proxyConfig, urls.lanUrlForConfig),
      host: HOST,
      port,
    };
    const devServer = new WebpackDevServer(serverConfig, compiler);
    // Launch WebpackDevServer.
    devServer.startCallback(() => {
      if (isInteractive) {
        clearConsole();
      }

      if (env.raw.FAST_REFRESH && semver.lt(react.version, '16.10.0')) {
        console.log(
          chalk.yellow(
            `Fast Refresh requires React 16.10 or higher. You are using React ${react.version}.`
          )
        );
      }

      console.log(chalk.cyan('Starting the development server...\n'));
      openBrowser(urls.localUrlForBrowser);
    });

    ['SIGINT', 'SIGTERM'].forEach(function (sig) {
      process.on(sig, function () {
        devServer.close();
        process.exit();
      });
    });

    if (process.env.CI !== 'true') {
      // Gracefully exit when stdin ends
      process.stdin.on('end', function () {
        devServer.close();
        process.exit();
      });
    }
  })
  .catch(err => {
    if (err && err.message) {
      console.log(err.message);
    }
    process.exit(1);
  });

Which starts a Webpack server:

 const devServer = new WebpackDevServer(serverConfig, compiler);
    // Launch WebpackDevServer.
    devServer.startCallback(() => {

and then opens the browser:

openBrowser(urls.localUrlForBrowser);

while running npm run eject, you can find a config folder with all configuration files. Inside path.js file you can find the default path and if want you can change the default path there. Be careful while doing this since it is an irreversible process

Ref :http://blog.teamtreehouse.com/getting-started-create-react-app-tool