How do I add a custom region?
In your sub-theme's .info.yml file add the new region alongside the existing ones, e.g.

regions:
  header: Header
  content: Content
  sidebar_first: 'Sidebar first'
  footer: Footer
  custom_region: 'Custom region'

Then in your page.html.twig file render it like so:

{{ page.custom_region }}

Visit the Block layout admin page and assign some blocks to the region so it displays on the page.