Not really a tutorial but some tech talk.
Some text used and adapted from
“ENCOURAGING COLLABORATION THROUGH APP INVENTOR”
Katherine Kyle Feeney
AppyBuilder allows users to download the project files (or source file) for their projects, which can then be reloaded. The structure will look something like this:
The assets folders hold all files that have been uploaded to the project as an asset. If no assets are associated with a project then there will be no assets folder.
If you import an extension a folder named external_comps will be created inside the assets folder. The extension will be uploaded to the external_comps folder. There will be a folder created inside the external_comps folder for every extension.
The src/com/appybuilder/[username]/[example] folder holds a .blk file and a .scm file for every screen in the project. For example every project will have a Screen1.blk file and a Screen1.scm file. The Screen1.blk file holds all information about the blocks associated with Screen1 and the Screen1.scm file holds information about the layout of Screen1.
The youngandroidproject folder holds only one file, the project.properties file. This file holds basic information
about the project, such as name, main screen and file structure.
The new option of importing and exporting screens gives another fileformat. It’s extension is SCR and you can see the filestructure above. You can see that extensions and assets are not exported with the file.
Instead there is a file with an extension called YAIL added. The following is an explanation from Franklyn Turbak i found on the App Inventor Google Group: