Question: how does a user with tne auth level of "author" submit his content to an editor/publisher in Joomla?

Answer: Once logged in and selecting your news menu, an author will see a "New" tag above the selection of the news categories for your existing articles that will take them to the editor and allow them to write their news.

Question: But this allows authors to submit in news only, not in other categories...

Answer: It is the same for all the categories - Just create the appropriate link or if you already have a menu item pointing to a category that will suffice. For usermenu, create a new menu item: Submit - Content then select the section.