Nothing is public by default. Your profile page, which you can leave empty, is only viewable to those in the same institution but can be made public. Search engines cannot index your eportfolio unless it is made public.
You can create a portfolio using the collections feature and add pages to this collection:
There is no access to anything in your pages and collections until you set whether you only want friends or the public to have access:
You can set no access except those who have the secret link which you can send to classmates or instructor: