Pages in GovCMS hold Regions. Regions are defined in the theme (in code, in an info yaml file) and cannot be altered via the GovCMS administration backend.
The content is placed into Regions as Blocks. Some blocks are created by Content Editors, they are called content blocks. Other blocks are created by modules or Views and provide predefined content listings from the site (e.g. listing of events or publications) or from other systems (e.g. a Twitter block shows Tweets).
You may explore the default theme GitHub repository to review the available regions: https://github.com/govCMS/govcms8_uikit_starter/blob/8.x-1.x/govcms8_uikit_starter.info.yml#L8
A theme is a set of files that define the visual look and feel of your site. The core software and modules that run on your site determine which content (including HTML text and other data stored in the database, uploaded images, and any other asset files) is displayed on the pages of your site. The theme determines the HTML markup and CSS styling that wraps the content.
The default theme shipped with GovCMS 8 is the “GovCMS 8 UI-Kit Starter” at https://github.com/govCMS/govcms8_uikit_starter.
The theme defines what regions are available in your GovCMS site.