Step1: Buy the apps
Step2: Download source code from the link given in the mail
Step3: Set development environment (Android on windows/iOS on Mac OS)
Step4: Open Android/iOS workspace file (DO NOT open project file, building error might occur sometimes)
Step5: Build Bundled AAB file (Use the key store they provided or create it on your own) / Archive generic device (including upload procedure)
Step6: Upload your AAB file to your Google Play Console / Select your version on App Store Connect (It might take a while after you archived it in Step5.)
Step7: Complete the forms and submit them
Notes:
After you complete the above steps, you can start to customize your Android/iOS apps such as app name, URL, icon, version number and etc. Why? Because everything you do will affect the building/uploading/archiving failure if you were rookies just like me.
P.S. Convertify support is not very strong. If you ran into some problems, they would recommend you to use "publy.app" service to upload the app for you. Of cause, you need to pay for it.