When you generate an entity with drupal console you get a twig template for the content of the entity. This is not a page and not a theme. If you want to use a page template, that is specific to the entity, then you use a template name suggestion, for exampe page--myentity.html.twig. The content from the entity controller will end up like any other content inside of this page in the region content if you use the standard block layout, where the block "Main page content" is placed in this region.
You can remove anything else from the page template what you don't need and replace this with static html.