App bundles nowadays is the official publishing format for publishing apps on Play store. An app bundle (.aab) is replacement for the earlier format apk that was widely used for distribution. App bundles are signed binaries that organize your app’s code and resources into modules

Android ecosystem has come up with two separate formats – the split publishing format for publishing apps on app store and serving format – APK for serving app on device. Google play optimises the serving format (apk) by selectively removing resources and codes  for multiple screen densities, device configurations, chip architectures and  languages from the publishing format (.aab) and keeps only what is required and used on device. If a new language support is enable by user, the apk would be updated for this new language on device via google play support at run time.

There is an average reduction of 20 % size seen due to the new served apks using App Bundle. Since, Google play rearranges and repackages the app bundles into APKs which are encrypted by developer keys during their creation, it requires the developers to share their keys using app-signing service. App bundles are divided in to modules and code and resources for each module are organized similarly to what you would find in an APK— as each of these modules may be generated as separate APKs. Google Play then uses the app bundle to generate the various APKs ( split apks ) that are served to users, such as the base APK, feature APKs, configuration APKs, and (for devices that do not support split APKs) multi-APKs.

• Base APK: This APK contains code and resources that all other split APKs can access and provides the basic functionality for your app. When a user requests to download your app, this APK is downloaded and installed first. That’s because only the base APK’s manifest contains a full declaration of your app’s services, content providers, permissions, platform version requirements, and dependencies on system features. Google Play generates the base APK for your app from your project’s app (or base) module

• Configuration APKs: Each of these APKs includes native libraries and resources for a specific screen density, CPU architecture, or language. When a user downloads your app, their device downloads and installs only the configuration APKs that target their device. Each configuration APK is a dependency of either a base APK or feature module APK. That is, they are downloaded and installed along with the APK they provide code and resources for. Unlike the base and feature modules, you don’t create a separate module for configuration APKs.

• Feature module APKs: Each of these APKs contains code and resources for a feature of your app that you modularize using feature modules. You can then customize how and when that feature is downloaded onto a device

App Bundles especially works well when you design modules of your app in a way that you can deliver modules as per requirement. Developers can deliver app modules conditionally or on demand as needed by user, just that there will be delay for the module to get downloaded the first time. The dynamic delivery mechanism is especially useful in areas like games where you may want to download resources for the next level in game on demand ie only when the user clears previous levels. This is also called dynamic  asset delivery. Developer can make asset packs and dynamically deliver them. Assets takes max space  for games, the core logic doesnt take that much space.

The structure of an Android App Bundle (*aab)

• base/, feature1/, and feature2/: Each of these top-level directories represent a different module of your app. The base module for your app is always contained in a base directory of the app bundle. However, the directory for each feature module is given the name specified by the split attribute in the module’s manifest.

• asset_pack_1/ and asset_pack_2/: For large, graphically-demanding apps or games, you can modularize assets into asset packs. Asset packs are ideal for games due to their large size limits. You can customize how and when each asset pack is downloaded onto a device according to three delivery modes: install-time, fast-follow, and on-demand. All asset packs are hosted on and served from Google Play.

• BUNDLE-METADATA/: This directory includes metadata files that contain information useful for tools or app stores. Such metadata files may include ProGuard mappings and the complete list of your app’s DEX files. Files in this directory are not packaged into your app’s APKs.

• Module Protocol Buffer (*.pb) files: These files provide metadata that helps describe the contents of each app module to app stores, such as Google Play.

• manifest/: Unlike APKs, app bundles store the AndroidManifest.xml file of each module in this separate directory

• dex/: Unlike APKs, app bundles store the DEX files for each module in this separate directory.

• res/, lib/, and assets/: These directories are identical to those in a typical APK. When you upload your app bundle, Google Play inspects these directories and packages only the files that satisfy the target device configuration, while preserving file paths.

• root/: This directory stores files that are later relocated to the root of any APK that includes the module that this directory is located in. For example, the base/root/ directory of an app bundle may include Java-based resources that your app loads using Class.getResource() 

App Bundles are helpful in other on demand module delivery scenarios as well such as developer may wish to deliver the premium features module only if user is a premium user or if the developer may want to provide feature modules based on in-app purchases done by the user. Developers need to use play core library for on demand modules but for conditional modules or install time modules – they can do the logic on android studio and using configuration. Modularisation of app also helps with build times as modules can be built in parallel. Also rebuilds are much shorter because less interdependent code since changing one piece may not required entire rebuild but only rebuild of module