An open space to build whatever you want with blocks and full site editing. Block Builder is to Block Themes what BlankSlate (blankslate.me) is to Classic Themes. Learn more about the difference in the Theme Handbook (developer.wordpress.org/themes). Block Builder is very minimal, giving you a completely blank canvas to build your own templates on as needed. You can add dynamic content to your theme with Theme Blocks (wordpress.org/documentation/article/blocks-list/#theme-blocks).