Packages
Packages are the publishing format for widgets, snippets, and themes (elements). Users can get developed elements by installing published packages via WebWriter.
Technical View
Packages are npm packages exporting widgets (.js/.ts), snippets (.html), and themes (.css).
Built-in Packages
The only difference between built-in and all other packages is that built-in packages come pre-installed with the editor. Otherwise, they implement the same interface.
Author/User View
To authors, packages are often synonymous with the main widget exported. They install/update/uninstall packages from WebWriter’s package manager. To users, packages are no concern at all - they only interact with the widgets and themes bundled into explorables.