When you create new content in a synced category or delete content from a synced category, the behavior depends on which knowledge base you created the content from.
When you first create a shared content category, that shared content category is considered a child. The original category it's synced to is considered the parent.
For content creation and deletion, the general rule of thumb is: if it happens in the parent, it's fully automatically synced. If it happens in the child, it isn't. (Note that this rule doesn't apply when you're editing existing synced content in other ways. In those cases, the changes are synced regardless of where they're made.)
So, for example, if you create a new subcategory or article from the parent category, that content is automatically created and synced in the child shared content category.
If you create a new subcategory or article in the child shared content category, that content only exists in the child.
Similarly, if you delete a subcategory or article from the parent category, that content is also automatically deleted in the child shared content category.
If you delete a subcategory or article from the child shared content category, that delete only impacts the child. The parent retains its copies of the subcategories or articles, and just loses the synced relationship to the child's deleted content.
The exception to these rules is if you unlink the content first. If you explicitly sever the syncing between subcategories or articles before you make edits, then those edits only impact the knowledge base where you make them.