How to I retrieve the ID from the Posts page?

You can use get_queried_object_id() to retrieve the page ID.

$custom_seo_desc = \get_post_meta( get_queried_object_id(), 'custom_page_desc' );

You can verify the page for posts ID by looking at the option page_for_posts.

$posts_page_id = get_option( 'page_for_posts' );

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