Did the bootstrap process change?
The process is rather different. There are some common elements but maybe what can get confusing is how D8 leverages the Symfony Framework.

use Drupal\Core\DrupalKernel;
use Symfony\Component\HttpFoundation\Request;

$autoloader = require_once 'autoload.php';

$kernel = new DrupalKernel('prod', $autoloader);

$request = Request::createFromGlobals();
$response = $kernel-&gt;handle($request);
$response-&gt;send();

$kernel-&gt;terminate($request, $response);


As can be witnessed by the index.php, things look a bit different. But it is essentially "bootstrapping" functionality. To get a really good sense of what is going on, I would highly recommend reading the following:

https://cipix.nl/understanding-drupal-8-part-1-general-structure-framework

Also, the Drupal 8 Documentation helps to get a good grasp on what is available to work with:

https://www.drupal.org/docs/8/

Especially giving the API's a good looksy:

https://www.drupal.org/docs/8/api

As I said, it's quite different than before. A lot more OOP going on. It's quite powerful and gives developers a lot more tools to work with but if you aren't used to how PSR-0 and PSR-4 autoloading work, you might find it hard to debug and grock. 

Good luck and Happy Drupalling!