explain structure of UI bundle in more detail

docs/modules/ROOT/pages/index.adoc

@@ -170,8 +170,13 @@ Since these optimizations are only applied to the pre-compiled files, they don't
 
 == UI compilation and generator consumption overview
 
-The purpose of an Antora UI project is to get the UI files into a state that Antora can use and to make it reusable.
-The only required file in the UI bundle is the default layout for pages (i.e., [.path]_layouts/default.hbs_) (and the layout for the 404 page, [.path]_layouts/404.hbs_ if the 404 page is enabled).
+The purpose of an Antora UI project is to assemble the UI files into a reusable distribution that Antora can use to compose the HTML pages and the assets they require.
+
+The only required file in the UI bundle is the default Handlebars layout for pages (i.e., [.path]_layouts/default.hbs_).
+If the 404 page is enabled, the Handlebars layout for the 404 page is also required (i.e., [.path]_layouts/404.hbs_).
+
+The layout files must be located in the [.path]_layouts_ folder in the UI bundle.
+The name of the layout is the stem of the file, which is the file's basename with a file extension (e.g., [.path]_layouts/default.hbs_ becomes `default`).
 
 [.output]
 ....
@@ -180,7 +185,28 @@ layouts/
   default.hbs
 ....
 
-Any additional files are only required because they are used by this layout, either when generating the HTML or assets referenced by the HTML that are served to the browser.
+There are no other required files in a UI bundle.
+Any additional files are only required because they are referenced by a layout, either when generating the HTML (partial or helper) or assets referenced by the HTML (CSS or JavaScript) that are served to the browser.
+Antora does not copy layouts, partials, or helpers to the generated site.
+
+If the layout looks for a partial, that partial must be located in the [.path]_partials_ directory.
+The name of the partial is the stem of the file (e.g,. [.path]_partials/body.hbs_] becomes `body` and used as `> body`).
+If the partial is inside a folder, the name of that folder is not used in the partial's name.
+Additionally, any JavaScript files found in the [.path]_helpers_ directory are automatically registered as template helpers.
+The name of the helper function matches the stem of the file (e.g., [.path]_heleprs/concat.js_ becomes `concat`).
+
+Here's how a UI would be structured if it had layouts, partials, and helpers:
+
+[.output]
+....
+helpers/
+  concat.js
+layouts/
+  404.hbs
+  default.hbs
+partials/
+  body.hbs
+....
 
 The UI is served statically in a production site, but the UI's assets live in a source form in a UI project to accommodate development and simplify maintenance.
 When handed off to the Antora pipeline, the UI is in an interim, pre-compiled state.