Site Studio Sync includes a source type of default_module_package. This type is handled by  the modules/cohesion_sync/src/Services/DefaultModulePackage.php service. Package source type services are gathered by PackageSourceManager service collector using sitestudio_package_source tag. It is possible to add your own custom types that would handle default packages in a different manner, for example fetching package stored in a remote FTP or AWS S3 bucket. To do so, you would need to follow these steps:

• Write your own package source service that Implements Drupal\cohesion_sync\Services\PackageSourceServiceInterface interface.
• Register your service using sitestudio_package_source tag and appropriate priority, depending if you are registering a new source type or overriding an existing one.
• Define your site_studio.packages.yml containing your new (or overridden) type and relevant metadata in source property.

More information about service collection and service tags is available here.

Customising package source types