The blog's authors, categories and tags are created here.
A blog author is not an admin user
An author is a display profile: a name, a bio and a picture shown on the site. It is not an account and grants nobody a login.
So a post can be credited to a guest or a colleague who never signs in to the admin.
Categories and tags are the same thing used differently
Both group posts and both produce their own archive page. The difference is convention, not mechanism: categories for the main structure, tags for scattered subjects.
Deleting a category does not delete posts
Removing a category or tag leaves the posts alone and only breaks their association with it. But that archive page no longer exists — if you linked to that address anywhere, the link breaks.