Regions Override, available from the upper right corner of your admin panel on all page types or under Edit » Additional Options » Page Layout panel, gives you the ability to strip out pre-configured page regions on a page-by-page basis. Removing certain regions can be beneficial especially if using Layout Builder or wishing to take advantage of all available space when presenting output from a View, all without having to implement any subtheming to achieve the same effect.

Layout Builder is a Drupal Core add-on feature designed to help users create more complex, visually appealing page layouts. This feature is entirely optional, but does require users to review the documentation to fully appreciate and learn its particular requirements and quirks.

Like a far more robust version of the Layout Column Widget which works within the Main Content block, the Layout Builder:

Understanding Layout Builder's configuration and structure is key to learning what's available and how to use this feature.

Now, let's identify Layout Builder's sections and their purpose.