How to display (plugin?) output on a single page (not a widget, probably not a short code) – template, plugin, combination?

In other common PHP frameworks you normally propagate a new route and have a controller as callback print the desired output. In WordPress something like that doesn’t seem to be built in – at the moment.

But what I’d recommend is to somehow mimic this approach.

1. In a custom plugin have a rule that when a certain custom URL is being called a certain custom template will be used to render the main content.
2. In that plugin provide a function (or multiple functions) that takes care of the business logic and prepares the desired output.
3. Inside the certain custom template from the first step call that function.

For example.

<?php
/*
Plugin Name: My Plugin
Description: Lorem ipsum dolor sit amet.
Version: 1.0
Author: You
Author URI: https://example.com
*/

class MyPlugin {

  public function __construct() {

    add_action('template_include', [$this, 'template_suggestion'];
  }

  function template_suggestion($template) {

    global $wp;

    $current_slug = $wp->request;

    $custom_template = locate_template(['foobar.php']);

    if ($current_slug == 'lorem-ipsum' && $custom_template != '') {

      status_header(200);
      return $custom_template;
    }

    return $template;
  }

  public static function Foo() {

    // Do whatever you need to do to build your markup.
    $output="<div>";
    $output .= 'Foo';
    $output .= '</div>';

    return $output;
  }

  public static function Bar() {

    // Do whatever you need to do to build your markup.
    $output="<div>";
    $output .= 'Bar';
    $output .= '</div>';

    return $output;
  }

}

new MyPlugin();

And then inside foobar.php simply:

<div>
  <?php echo MyPlugin::Foo();?>
  <?php echo MyPlugin::Bar();?>
</div>