Set the template for the frontpage How do I change the Drupal 8 frontpage template, so it doesn't use the regular page.html.twig template?
Enable twig_debug and you will see HTML comments surrounding the output that tell you what template suggestions are available. In this case you will get output like:

&lt;!-- THEME DEBUG --&gt;
&lt;!-- THEME HOOK: 'page' --&gt;
&lt;!-- FILE NAME SUGGESTIONS:
   * page--front.html.twig
   * page--node.html.twig
   x page.html.twig
--&gt;
&lt;!-- BEGIN OUTPUT from 'core/modules/system/templates/page.html.twig' --&gt;


Naming your template page--front.html.twig will use that template for only the homepage.