page.html.twig default regions
If no regions are specified in the theme, the core provides defaults itself.

From the docs page: https://www.drupal.org/docs/8/theming-drupal-8/adding-regions-to-a-theme


  Default Regions:
  
  page.header: Items for the header region.
  
  page.primary_menu: Items for the primary menu region.
  
  page.secondary_menu: Items for the secondary menu region.
  
  page.highlighted: Items for the highlighted content region.
  
  page.help: Dynamic help text, mostly for admin pages.
  
  page.content: The main content of the current page.
  
  page.sidebar_first: Items for the first sidebar.
  
  page.sidebar_second: Items for the second sidebar.
  
  page.footer: Items for the footer region.
  
  page.breadcrumb: Items for the breadcrumb region.
  
  If your theme does not declare any regions Drupal will assume this set
  of defaults. These regions correspond with what the default
  core/modules/system/templates/page.html.twig file expects, as well as
  two hidden regions, page_top, and page_bottom - you don't need to
  declare these final two if you override the defaults, however the {{
  page_top }} and {{ page_bottom }} Twig variables should be retained
  in the html.html.twig template.