Q:

Custom Theme - Add page specific meta tags / title to pages

I am new to Drupal, but I am very familiar with WordPress. 
In WordPress, I am able to hook into the head of a document with wp_head to add things like page specific meta tags. 
When using a custom theme based on Stable, how would you add page specific things in the <head> of a document in Drupal 8? 
Note: I am wondering how this would be done without a custom module. 

A:

You can attach the tag to the page's render array via hook_page_attachments_alter().  
An example from the related change record:
function MYTHEME_page_attachments_alter(array &$page) {
    $viewport = array(
        '#type' => 'html_tag',
        '#tag' => 'meta',
        '#attributes' => array(
            'name' => 'viewport',
            'content' => 'width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no',
        ),
    );
    $page['#attached']['html_head'][] = [$viewport, 'viewport'];
}

