How to display the maximum upload size in a WordPress single site?

The answer actually lies on the same page where you inspected your modifications: /wp-admin/media-new.php.

If you get to the /wp-admin/media-new.php file, you will get a function:

<?php media_upload_form(); ?>

And the function is located in /wp-admin/includes/media.php, where you will get the following line:

$max_upload_size = wp_max_upload_size();

So the holly grail you are searching is: wp_max_upload_size().

So you can easily follow the core, and display the maximum uploaded size anywhere with the following bit of code (thanks to the core):

<?php
$max_upload_size = wp_max_upload_size();
if ( ! $max_upload_size ) {
    $max_upload_size = 0;
}

/* translators: %s: Maximum allowed file size. */
printf( __( 'Maximum upload file size: %s.', 'wpse355118' ), esc_html( size_format( $max_upload_size ) ) );
?>