In Drupal 8 can a developer inject Full HTML Pages?
Usually you will not inject full HTML pages to Drupal, more likely you will inject your content as node-content. Then use a convenient theme for your needs.

Drupal has - like wordpress - themes. These are nested pieces of HTML code with TWIG syntax for dynamic content.

Unlike, wordpress, you can specify different themes for different URI paths (look for keyword theme_negotiator). I.e., you may configure to use an "article-theme" on path /articles.

But this is only the beginning of a very long story...