Click the one you want to insert in the text. If your document has many headings, it's a good idea to add a table of contents in case it will be accessed outside Google Docs, for instance, if you ...

You can create & add a document outline in Google Docs using this guide. Difference between Table of contents & Document Outline explained.

How to make different headers for odd and even pages in Word In long documents, especially when you’re viewing a two-page spread, it’s common to have a different header on odd and even pages.