By having a low level indexing functionality, modules in the domain layer can subscribe to search functionality and also offer their content to the global website search.
This module controls the Sitecore item serialisation, e.g. the ability to version control content in Sitecore.
The Sitecore extensions module contains a range of simple extensions to the Sitecore platform to optimize development.
The Theming module contains base theming and front-end assets for the solution, including Bootstrap and misc plug-ins.