Twitch host the frontend of extensions on their CDN, so when your extension goes to review you zip just the files inside your build folder, so your HTML, JS, CSS etc…, and those will be reviewed by Twitch.
The backend you can host however you want, as it’s your responsibility to host it and ensure it complies with the extension guidelines (such as only communicating with the frontend over SSL, so HTTPS or WSS). What commands you use to run that, or what software or hosts you use to run that is entirely up to you.