Just display content between shortcode brackets

I have good news: the answer is simple.

The WordPress Codex concerning Enclosing Shortcodes (like the one you posted here) shows that the shortcode callback has 2 arguments, $atts and $content. You want to work with $content:

function wporg_shortcode($atts = [], $content = null)
{
    // do something to $content

    // always return
    return $content;
}
add_shortcode('wporg', 'wporg_shortcode');