Adding fields to content type edit form (not to the node itself)

In Drupal 8, how do you add fields to the content type edit form, i.e. where you define the content type? I know how to add fields to the content type that wind up on the node edit form, but I’m looking to add a field to the content type itself. Essentially, what I’m trying to do is add some data to content types that would allow me to group them together. To do this, I need to add some additional metadata to the content type beyond name and description.