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In WordPress settings you can set a page as the Posts Page. This can be found:
WP-Admin->Settings->Reading Settings->Front page displays
How do I retrieve the page ID that is set in this setting?
Use the page_for_posts option:
$page_for_posts = get_option( 'page_for_posts' );
This will return the ID of the Page assigned to display the Blog Posts Index.
Required, but never shown