- 🇺🇸United States mortona2k Seattle
This module only generates the migration config, which is a highly recommended way to upgrade.
Modules can have migration source plugins, which the generated migrations will use. See Drupal\node\Plugin\migrate\source\d7\NodeComplete.
Contrib modules can have migration plugins, see the address module which has a MigrateField plugin.
I think what happens is that when migrate drupal defines the migrations to be exported, it looks up the field type and finds migration plugins that support it. See Drupal\migrate_drupal\FieldDiscovery::addBundleFieldProcesses().