To create a new page, you should select "Pages" from the menu on the left and select "Add New" from the sub-menu. To edit a page click on "All pages" and then select the particular one.
It is always best to use "Avada Builder" or "innotheme Builder". This will allow you to use the page structure with containers and columns.
On the right side you can select the parent page under "Page Attributes" at "Parent". At "Template" you can set "100% width" if necessary to display the page contents on the complete screen width. If you want to display content on the entire width of the page, you must also click on the "Brush" in the blue area of the respective container and change the "Interior Content Width" to "100% Width".