Adding bundle subclass to node types in Drupal 9 and modern Drupal

There is no good example in Drupal core:

There is no example in the Examples project (@TODO file issue)

And the change record – Introducing bundle classes | – and the definitive blog post by one of the main authors of the functionality – Adding Bundle Subclasses to Drupal Core 9.3.0 (Part 2) - TEN7 – seem to be lacking some crucial information.

Mainly, how does a given content type get told to inherit a trait in the first place?

(We are not talking about the define keyword specified constant, but rather the

Yes, you can add a class constant as an array, but for traits these can only be added to the Interface that the trait implements— do not try to add to the Trait class directly.