Portfolio page is just an intermediate page which loops portfolio items. So you cannot edit your Portfolio page by default with Cornerstone or Pro content builder. Once you create a default WordPress page, you have the option to select the portfolio layout under "page attributes" section and then it will show all the portfolio items in that page.
Further options related to Portfolio page can be accessible under "Portfolio Settings" when editing your page using WordPress.
If you want to modify some of the elements in the portfolio page, you will have to do it by adding custom css or by modifying the Layout - Portfolio page templates instead.
For further information with regard to Portfolio section, please check our knowledge base articles below.
Hope that helps.