A service is a class that has functions to use in another class or module. This provides an interface for developers to access foreign code in a clean way.
Modules load services from the Drupal utility class.
$field_type_manager = \Drupal::service('plugin.manager.field.field_type');
Classes load services using dependency injection so the class can be tested.
Services are defined in *.services.yml files. This is where the "plugin.manager.field.field_type" service name is defined.
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