Ajax - How to create a Category through actionTrigger
Of course there is, otherwise we would not be able to store categories in Craft. The controller that handles categories is called CategoriesController and it contains the function actionSaveCategory so you route will be actions/categories/saveCategory

To create/save a user you have the UsersController.. You may guess the function is actionSaveUser => actions/users/saveUser 

It's the same for every element in Craft. EntriesController, actionSaveEntry => actions/entries/saveEntry

But this all has nothing to do with an endpoint or an API, it's just basic Craft code