Inside of your app -> views -> layouts directory, create your layout file... In this case, assume layout.phtml.
Inside of your "MyController" controller, make sure you have this:
    public function initialize() {
        $this->view->setTemplateAfter('layout');
    }
This will tell the controller to use the layout template.
Inside of your layout file, make sure you have this where you want the included content to be placed:
    <?php echo $this->getContent(); ?>
Then when you resolve a route, you'll do this:
    $router->addGet("/some-path",[
        'controller' => 'mycontroller',
        'action' => 'somepath'
    ]);
Then inside of your "MyController" controller you'll do this:
    public function somepathAction() {
        $this->view->pick('some-path');
    }
At this point you'll have a file in app->views->some-path.phtml (or volt or whatever) that will have the content you want rendered inside of the layout.