public BaseFieldDefinition::getTargetBundle()
Gets the bundle the field is attached to.
This method should not be confused with EntityInterface::bundle() (configurable fields are config entities, and thus implement both interfaces):
FieldDefinitionInterface::getTargetBundle() answers "as a field, which bundle are you attached to?". EntityInterface::bundle() answers "as a (config) entity, what is your own bundle?" (not relevant in our case, the config entity types used to store the definitions