Docs: Update parallel routes docs, add default.js pt 1. (vercel#60806)

docs/01-getting-started/02-project-structure.mdx

@@ -76,7 +76,7 @@ This page provides an overview of the file and folder structure of a Next.js pro
 
 |                                                                                                |                            |
 | ---------------------------------------------------------------------------------------------- | -------------------------- |
-| [`@folder`](/docs/app/building-your-application/routing/parallel-routes#convention)            | Named slot                 |
+| [`@folder`](/docs/app/building-your-application/routing/parallel-routes#slots)                 | Named slot                 |
 | [`(.)folder`](/docs/app/building-your-application/routing/intercepting-routes#convention)      | Intercept same level       |
 | [`(..)folder`](/docs/app/building-your-application/routing/intercepting-routes#convention)     | Intercept one level above  |
 | [`(..)(..)folder`](/docs/app/building-your-application/routing/intercepting-routes#convention) | Intercept two levels above |

docs/02-app/01-building-your-application/01-routing/10-parallel-routes.mdx

@@ -1,11 +1,14 @@
 ---
 title: Parallel Routes
 description: Simultaneously render one or more pages in the same view that can be navigated independently. A pattern for highly dynamic applications.
+related:
+  links:
+    - app/api-reference/file-conventions/default
 ---
 
-Parallel Routing allows you to simultaneously or conditionally render one or more pages in the same layout. For highly dynamic sections of an app, such as dashboards and feeds on social sites, Parallel Routing can be used to implement complex routing patterns.
+Parallel Routes allows you to simultaneously or conditionally render one or more pages within the same layout. They are useful for highly dynamic sections of an app, such as dashboards and feeds on social sites.
 
-For example, you can simultaneously render the team and analytics pages.
+For example, considering a dashboard, you can use parallel routes to simultaneously render the `team` and `analytics` pages:
 
 <Image
   alt="Parallel Routes Diagram"
@@ -15,33 +18,9 @@ For example, you can simultaneously render the team and analytics pages.
   height="952"
 />
 
-Parallel Routing allows you to define independent error and loading states for each route as they're being streamed in independently.
+## Slots
 
-<Image
-  alt="Parallel routes enable custom error and loading states"
-  srcLight="/docs/light/parallel-routes-cinematic-universe.png"
-  srcDark="/docs/dark/parallel-routes-cinematic-universe.png"
-  width="1600"
-  height="1218"
-/>
-
-Parallel Routing also allows you to conditionally render a slot based on certain conditions, such as authentication state. This enables fully separated code on the same URL.
-
-<Image
-  alt="Conditional routes diagram"
-  srcLight="/docs/light/conditional-routes-ui.png"
-  srcDark="/docs/dark/conditional-routes-ui.png"
-  width="1600"
-  height="898"
-/>
-
-## Convention
-
-Parallel routes are created using named **slots**. Slots are defined with the `@folder` convention, and are passed to the same-level layout as props.
-
-> Slots are _not_ route segments and _do not affect the URL structure_. The file path `/@team/members` would be accessible at `/members`.
-
-For example, the following file structure defines two explicit slots: `@analytics` and `@team`.
+Parallel routes are created using named **slots**. Slots are defined with the `@folder` convention. For example, the following file structure defines two slots: `@analytics` and `@team`:
 
 <Image
   alt="Parallel Routes File-system Structure"
@@ -51,7 +30,7 @@ For example, the following file structure defines two explicit slots: `@analytic
   height="687"
 />
 
-The folder structure above means that the component in `app/layout.js` now accepts the `@analytics` and `@team` slots props, and can render them in parallel alongside the `children` prop:
+Slots are passed as props to the shared parent layout. For the example above, the component in `app/layout.js` now accepts the `@analytics` and `@team` slots props, and can render them in parallel alongside the `children` prop:
 
 ```tsx filename="app/layout.tsx" switcher
 export default function Layout(props: {
@@ -81,19 +60,28 @@ export default function Layout(props) {
 }
 ```
 
-> **Good to know**: The `children` prop is an implicit slot that does not need to be mapped to a folder. This means `app/page.js` is equivalent to `app/@children/page.js`.
+However, slots are **not** [route segments](/docs/app/building-your-application/routing#route-segments) and do not affect the URL structure. For example, for `/dashboard/@analytics/views`, the URL will be `/dashboard/views` since `@analytics` is a slot.
+
+> **Good to know**:
+>
+> - The `children` prop is an implicit slot that does not need to be mapped to a folder. This means `app/page.js` is equivalent to `app/@children/page.js`.
+
+## Active state and navigation
 
-## Unmatched Routes
+By default, Next.js keeps track of the active _state_ (or subpage) for each slot. However, the content rendered within a slot will depend on the type of navigation:
 
-By default, the content rendered within a slot will match the current URL.
+- [**Soft Navigation**](/docs/app/building-your-application/routing/linking-and-navigating#5-soft-navigation): During client-side navigation, Next.js will perform a [partial render](/docs/app/building-your-application/routing/linking-and-navigating#4-partial-rendering), changing the subpage within the slot, while maintaining the other slot's active subpages, even if they don't match the current URL.
+- **Hard Navigation**: After a full-page load (browser refresh), Next.js cannot determine the active state of slots that don't match the current URL. Instead, it will render a [`default.js`](#defaultjs) file for the unmatched slots, or `404` if `default.js` doesn't exist.
 
-In the case of an unmatched slot, the content that Next.js renders differs based on the routing technique and folder structure.
+> **Good to know**:
+>
+> - The `404` for unmatched routes helps ensure that you don't accidentally render a route that shouldn't be parallel rendered.
 
 ### `default.js`
 
-You can define a `default.js` file to render as a fallback when Next.js cannot recover a slot's active state based on the current URL.
+You can define a `default.js` file to render as a fallback for unmatched slots during the initial load or full-page reload.
 
-Consider the following folder structure. The `@team` slot has a `settings` directory, but `@analytics` does not.
+Consider the following folder structure. The `@team` slot has a `settings` page, but `@analytics` does not.
 
 <Image
   alt="Parallel Routes unmatched routes"
@@ -103,19 +91,15 @@ Consider the following folder structure. The `@team` slot has a `settings` direc
   height="930"
 />
 
-#### Navigation
+When navigating to `/dashboard/settings`, the `@team` slot will render the `settings` page while maintaining the currently active page for the `@analytics` slot.
 
-On navigation, Next.js will render the slot's previously active state, even if it doesn't match the current URL.
+On refresh, Next.js will render a `default.js` for `@analytics`. If `default.js` doesn't exist, a `404` is rendered instead.
 
-#### Reload
+Additionally, since `children` is an implicit slot, you also need to create a `default.js` file to render a fallback for `children` when Next.js cannot recover the active state of the parent page.
 
-On reload, Next.js will first try to render the unmatched slot's `default.js` file. If that's not available, a 404 gets rendered.
+### `useSelectedLayoutSegment(s)`
 
-> The 404 for unmatched routes helps ensure that you don't accidentally render a route that shouldn't be parallel rendered.
-
-## `useSelectedLayoutSegment(s)`
-
-Both [`useSelectedLayoutSegment`](/docs/app/api-reference/functions/use-selected-layout-segment) and [`useSelectedLayoutSegments`](/docs/app/api-reference/functions/use-selected-layout-segments) accept a `parallelRoutesKey`, which allows you to read the active route segment within that slot.
+Both [`useSelectedLayoutSegment`](/docs/app/api-reference/functions/use-selected-layout-segment) and [`useSelectedLayoutSegments`](/docs/app/api-reference/functions/use-selected-layout-segments) accept a `parallelRoutesKey` parameter, which allows you to read the active route segment within a slot.
 
 ```tsx filename="app/layout.tsx" switcher
 'use client'
@@ -142,7 +126,7 @@ export default function Layout(props) {
 }
 ```
 
-When a user navigates to `@auth/login`, or `/login` in the URL bar, `loginSegments` will be equal to the string `"login"`.
+When a user navigates to `app/@auth/login` (or `/login` in the URL bar), `loginSegments` will be equal to the string `"login"`.
 
 ## Examples
 
@@ -291,6 +275,16 @@ export default function CatchAll() {
 
 ### Conditional Routes
 
+Parallel Routes also allows you to conditionally render a slot based on certain conditions, such as authentication state. For example, you can render a `/dashboard` or `/login` page depending on whether the user is logged in:
+
+<Image
+  alt="Conditional routes diagram"
+  srcLight="/docs/light/conditional-routes-ui.png"
+  srcDark="/docs/dark/conditional-routes-ui.png"
+  width="1600"
+  height="898"
+/>
+
 Parallel Routes can be used to implement conditional routing. For example, you can render a `@dashboard` or `@login` route depending on the authentication state.
 
 ```tsx filename="app/layout.tsx" switcher
@@ -324,3 +318,15 @@ export default function Layout({ dashboard, login }) {
   width="1600"
   height="898"
 />
+
+### Streaming
+
+Parallel Routes can be streamed independently, allowing you to define independent error and loading states for each route:
+
+<Image
+  alt="Parallel routes enable custom error and loading states"
+  srcLight="/docs/light/parallel-routes-cinematic-universe.png"
+  srcDark="/docs/dark/parallel-routes-cinematic-universe.png"
+  width="1600"
+  height="1218"
+/>

docs/02-app/02-api-reference/02-file-conventions/default.mdx

@@ -1,6 +1,32 @@
 ---
 title: default.js
 description: API Reference for the default.js file.
+related:
+  title: Learn more about Parallel Routes
+  links:
+    - app/building-your-application/routing/parallel-routes
 ---
 
-This documentation is still being written. Please check back later.
+The `default.js` file is used to render a fallback within [Parallel Routes](/docs/app/building-your-application/routing/parallel-routes) when Next.js cannot recover a [slot's](/docs/app/building-your-application/routing/parallel-routes#slots) active state after a full-page load.
+
+During [soft navigation](/docs/app/building-your-application/routing/linking-and-navigating#5-soft-navigation), Next.js keeps track of the active _state_ (subpage) for each slot. However, for hard navigations (full-page load), Next.js cannot recover the active state. In this case, a `default.js` file can be rendered for subpages that don't match the current URL.
+
+Consider the following folder structure. The `@team` slot has a `settings` page, but `@analytics` does not.
+
+<Image
+  alt="Parallel Routes unmatched routes"
+  srcLight="/docs/light/parallel-routes-unmatched-routes.png"
+  srcDark="/docs/dark/parallel-routes-unmatched-routes.png"
+  width="1600"
+  height="930"
+/>
+
+When navigating to `/dashboard/settings`, the `@team` slot will render the `settings` page while maintaining the currently active page for the `@analytics` slot.
+
+On refresh, Next.js will render a `default.js` for `@analytics`. If `default.js` doesn't exist, a `404` is rendered instead.
+
+Additionally, since `children` is an implicit slot, you also need to create a `default.js` file to render a fallback for `children` when Next.js cannot recover the active state of the parent page.
+
+## Props
+
+`default.js` does not receive any props.