How to get current page title inside a page itself in a shortcode and in page content

You can create a shortcode to fetch the title of current page. Try copying the following snippet in your child theme’s functions.php

function post_title_shortcode(){
    return get_the_title();
}
add_shortcode('post_title','post_title_shortcode');

Use [post_title] shortcode to display the title of the post or page.