The Kobos don't have folders either.
But if you are using subfolders to organise the books by author/series then current Kobo firmware does this for you, so for example if you list or search books by author then the titles by that author are listed with each series grouped together and in proper series order.
The Kobos have shelves, which are esentially the same thing as collections or categories in that you can put a book on more than one shelf. So combining shelves with the series sorting it is easy to organise your books by category --> author --> series without the need for subfolders.
Of course if you have some other organisation in mind then this won't help you.