How auto rebuild and reload browser works in create-react-app - reactjs

Question

hi I am a newbie in reactJS and am learning reactjs recently by kickstarting from the create-react-app and modifiying it . My doubt is that how the create-react-app detects the changes in files and autimatically rebuilds them . I used a normal nodejs + react + webpack app and in that i have to manually specify the npm run build and node start in the package.json like as
"scripts": {
"build": "webpack",
"start": "node server"
},
in case the above for the create-react-app differ as
"scripts": {
"predeploy": "npm run build",
"deploy": "gh-pages -d build",
"start": "npm run build-css && run-p -ncr watch-css start-js",
"start-js": "react-scripts start",
"build": "run-s -n build-css build-js",
"build-js": "react-scripts build",
"test": "run-s -n build-css test-js",
"test-js": "react-scripts test --env=jsdom",
"build-css": "node-less-chokidar src",
"watch-css": "node-less-chokidar src --watch",
"eject": "react-scripts eject"
}
What makes the create-react-app and a normal node+webpack+react app differ in rebuilding the source ? (Starting the server seems not a deal because once its started it will be runing on a port for both cases . For Server restart both apps needed to kill the process and start again . So building the source is my only concern) . Any help is appreciated .