Creating Templates
Templates (also known as "schemas") are reusable JSON-strings that can be used to create a bunch of the components with only few blocks. They are usually generated from existing screens of the project files (AIA files), but you can also write templates on your own.
1. Use Schema Generator
It is currently the only recommended way to generate templates. You will just need to create your design in your App Inventor / Kodular, then grab the project file and finally, import to the generator.
It is not a preferred method as it doesn't worth considering the time spent. But if you are still interested, see Creating Templates Manually for more information.