For creating an outline, the SERP Explorer option in a document is an option to use.
If you click the "Outline" tab and "Open SERP Explorer", you will see a horizontal view of the SERP results pulled for the query you entered to create the document initially.
Scroll horizontally to look through the sources, and click any of the headers on this view and they will be highlighted in green, and added to the window on your right. Also on this small window on the right, click "+add custom header" to type out your own header.
When done compiling headers, click the green "Save Outline" button. Also, filter through the headers and content by typing in the search bar ("filter by keyword") at the top of the window, and choose H2, H3, ect. from the "Heading Type" dropdown also at the top.