Add an ARCHITECTURE.md.

ARCHITECTURE.md

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+# Architecture
+
+This document describes the layout of the samples repository.
+
+The top level directory is a Gradle project. This directory can be opened in
+Android Studio (and that will be the easiest way to work with the samples).
+
+## Directory structure
+
+### Samples
+
+Most subdirectories are the individual sample apps. These can't be opened in
+Android Studio individually, as they rely on common code from the top level
+project. These subdirectories have their own `build.gradle` (the Groovy DSL) or
+`build.gradle.kts` (the Kotlin DSL) files. In Gradle terms these are "projects".
+Android Studio calls them "modules". The documentation in this repository will
+typically call them "modules".
+
+Each sample has its own README.md that explains what features it demonstrates,
+as well as any unique requirements.
+
+To run a sample, use the configuration selector in the top panel of Android
+Studio to select the sample and then click run. For example, to run the
+endless-tunnel sample game:
+
+![Select and run a sample][docs/run-sample.png]
+
+#### Build types
+
+Android Gradle modules have multiple "build types", sometimes called "variants".
+For most of the samples, there are only two build types: debug and release. A
+few samples, notably the sanitizers sample, have more. To view the build types
+for the modules in this repository, in the Android Studio application menu,
+select View -> Tool Windows -> Build Variants. A window will open that lets you
+select the active variant for each sample.
+
+### build-logic
+
+The `build-logic` directory contains Gradle convention plugins used by this
+repository. This is where Gradle policy decisions that apply to the whole
+repository are made.
+
+See the README.md in that directory for more details.
+
+### docs
+
+Documentation and supporting artifacts for this repository. Yes, for now it's
+just images for the READMEs and this doc.
+
+### gradle/libs.versions.toml
+
+This is a Gradle [version catalog]. It's the central location that defines the
+library and plugin dependencies for each sample.
+
+The Android Gradle Plugin does not support evaluating version catalog fields in
+its DSL, so versions for SDK and NDK tools (`ndkVersion`, `compileSdkVersion`,
+`targetSdkVersion`, etc) are all defined by the convention plugins in
+[build-logic](#build-logic).
+
+[version catalog]: https://docs.gradle.org/current/userguide/platforms.html
+
+### Gradle wrapper
+
+The `gradlew` and `gradlew.bat` scripts are the Gradle wrappers, macOS/Linux-
+and Windows batch-compatible respectively. They will manage the Gradle
+installation used by this repository for you, ensuring that the correct versions
+and environment are used. These should be used instead of running `gradle`
+directly.
+
+`gradle/wrapper/gradle-wrapper.properties` defines which version of Gradle will
+be used. This should rarely be modified manually. Android Studio's Upgrade
+Assistant will manage that file and upgrade to whatever version of Gradle it
+prefers for that version of Android Studio and Android Gradle.
+
+### build.gradle and settings.gradle
+
+The build.gradle (or build.gradle.kts) file is the top level build file. It
+doesn't do anything interesting but declares which plugins will be used by the
+child modules. Each sample's module has its own build.gradle file that defines
+properties of the app.
+
+The settings.gradle (or settings.gradle.kts) file configures repositories for
+fetching dependencies, and declares each app module.
+
+### Metadata directories
+
+#### .github
+
+This directory contains GitHub metadata files, such as GitHub Action workflows
+and Issue templates.
+
+#### .google
+
+The `packaging.yaml` file in this directory is used by Android Studio to enable
+the File -> New -> Import Sample feature. As this repository is a monolithic
+sample project, it should not need to be changed.

README.md

@@ -4,6 +4,9 @@
 
 This repository contains sample apps that use the [Android NDK].
 
+For an explanation of the layout of this repository, see
+[ARCHITECTURE.md](ARCHITECTURE.md).
+
 ## Build and run
 
 1. Clone the repository