How to load a post into an empty div tag anywhere across the pages?

Question

You can solve this in different ways.

One it’s to use simple querys with WP_Query and using the p parameter to retrive a particular post like it’s explained here.
Something like this:

$the_query = new WP_Query( 'p=123' );
while ( $the_query->have_posts() ) : $the_query->the_post();
    the_content();
endwhile;

Where 123 it’s the post id from where you need the data (the_content).

Or you can query the content from a single post using get_post function.
Like this:

$postid = 123;
$queryp = get_post($postid);
$content = $queryp->post_content;
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]&gt;', $content);
echo $content;

Where 123, again, it’s the postid from where you need to extract the content.

And i think wp_get_single_post function should work too.

Then you can control your content everywhere just extracting the desired data from certains posts by postid