How can I show the actual content of Posts page because the_content() is showing all blog content?

You can use:

1. get_queried_object_id() to get the ID of the static posts Page. Alternatively, you may also use get_option( 'page_for_posts' ).

   $post_id = get_queried_object_id();
   //$post_id = get_option( 'page_for_posts' ); // you should use the above on the blog page
   $content = get_the_content( null, false, $post_id );
   

2. get_queried_object() to get the full data of the static posts Page. (You’d get an object just as the one returned by get_post().)

   $post = get_queried_object();
   $content = get_the_content( null, false, $post ); // or you could use $post->post_content
   

And on the posts page, the global $wp_query (the main query) already contains the latest/blog posts, so there is no need to make a secondary query like the $wpb_all_query in your code. Just use the main loop to display the posts and you can use the pre_get_posts hook to alter the main query, e.g. to change the number of posts queried per page.

So your home.php template could be as simple as:

<?php get_header(); ?>

<h1><?php single_post_title(); ?></h1>

<?php
// Display the content of the static posts Page.
// This is just an example using setup_postdata().
$post = get_queried_object();
setup_postdata( $post );
the_content();
wp_reset_postdata();
?>

<?php
// Display the latest/blog posts.
if ( have_posts() ) :
    while ( have_posts() ) : the_post(); ?>
        <h2><?php the_title(); ?></h2>
    <?php
    endwhile;
endif;
?>

<?php get_footer(); ?>