Add the following code inside your theme’s functions.php file to get WordPress post content by post id.
$my_postid = 12;//This is page id or post id
$content_post = get_post($my_postid);
$content = $content_post->post_content;
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]>', $content);
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