GrowERP consist completely out of a number of plugins which are called by an application. The application is just a list of screens and a menu structure. An example is the 'admin' application which shows all the functions currently available within the system. All packages names which start with 'growerp' are plugins.
A plugin gives you the ability to create or update a system functionlity independent of the existing system based on other plugins which are froozen in the https://pub.dev repository.
It is always your own decision if you would like to upgrade by updating the plugins you base your plugin on in the pubspec.yaml file.
Further, plugins can be maintained and tested outside of the existing system. When finished, the existing system can be upgraded by changing the plugin version in the pubspec.yml.
A GrowERP plugin can be as simple as a single screen, or a number of screens together:
In general you store your files in the src directory so they will not be visble to its users. The files you want to make visble are exported in the dart file with the same name as the plugin package.
plugin_name lib plugin_name.dart src models views widgets