Customized frontend web page for plugin with no ‘bodily’ web page?

I must create a frontend web page (and sub-pages) which is able to show customized knowledge that’s saved within the database however will not be a part of wordpress, it isn’t a submit or web page as such however API referred to as knowledge. I don’t need to use a brief code due to the necessity of sub-pages.

I’ve appeared on the following however do not appear to have the ability to make it work as required: Dynamic URL, not a bodily web page inside the database

What I want is to have for instance website.com/mypage/ however I are not looking for ‘mypage’ to exist within the backend.

I’ve achieved this with:

public perform mypage_rewrite_ext() {
    international $wp_rewrite;
    $plugin_url = plugins_url( 'mypage.php', __FILE__ );
    $plugin_url = substr( $plugin_url, strlen( home_url() ) + 1 );
    // The sample is prefixed with '^'
    // The substitution is prefixed with the "dwelling root", not less than a '/'
    // That is equal to appending it to `non_wp_rules`
    $wp_rewrite->add_external_rule( 'mypage$', $plugin_url );
}

However I want my web page to nonetheless be inside WordPress as I want to make use of the header and footer.

I’ve tried the next from the above linked query, however this simply redirects to the house web page (even after re-saving permalinks):

public perform add_query_vars( $query_vars )
{
    $query_vars[] = 'check';
    return $query_vars;
}

public perform add_endpoint()
{
    add_rewrite_rule('^mypage/?', 'index.php?__test=1', 'high');
    flush_rewrite_rules(false); //// <---------- REMOVE THIS WHEN DONE
}

public perform sniff_requests($wp_query)
{
    international $wp;

    if(isset($wp->query_vars[ '__test' ])) {
        add_filter('template_include', perform ($original_template) {

            // change the default template to a google map template
            return plugin_dir_path( __FILE__ ) . 'mypage.php';
        });
    }
}

Any concepts on 1) what I’m doing flawed and/or 2) how I can obtain this?

That is the outcome from the analyser: