There’s many methods to accomplish this many of which are fully automated. For us, after we build the web app using Grunt as the task runner, we use the Cordova CLI to build our applications (APK and IPA). Once you do that, you just have to upload it to the store. For Apple, we use Transporter which allows you to drag and drop an IPA for upload. For Google, we upload the APK to their website.
If you have any specific questions please let me know.
Thank you!