The Node entity type happens to have bundles which is the reason for which we have a bundle_label property as well:
* bundle_label = @Translation("Content type"),
We can see in the following that it states "I have bundles" by defining the ID of the plugin (which maps to a configuration entity type) of its bundles:
bundle_entity_type = "node_type",
Lo and behold, that is the NodeType's ConfigEntityType plugin ID. On its plugin annotation, we can find the reverse bundle_of property which references the Node entity type. Needless to say, this is not mandatory for all configuration entity types but used for the ones that act as content entity bundles. For example, the View configuration entity type does not have this.
In addition, we ...