The heart of all WordPress Themes are Templates. They are automatically assigned according to the WordPress Template Hierarchy based on file names in the Theme.
Here you can assign your Layout Templates to be used as Templates in your Theme. You assign one Template for each Part of your Site Architecture, or they will fall back on a Default Template corresponding to the type of Template needed for that Part.
For each Part you can either Change Template, Edit the existing Template (in the Layout Builder) or create a New Template using the Layout Builder.