Each page of your site is generated by the theme. Some pages, such as single Post or single Page, are made almost entirely of the content you enter in the editor. The rest are lists or search results or the 404 page, which is not anything that you edit in the editor. The theme pulls the pieces together to make the pages.
If you go to your index page and use your browser menu to save it as HTML, you could then paste that into a new page in the editor. But you shouldn’t have header images or menus in your page, and it would only be a static copy, not changing to include each new post your write.
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