Android Archives are a compiled version of Android Libraries, and are the recommended way to format plug-ins that you want to distribute. However, while you create a plug-in, it’s faster to work with the Android Library format since that doesn’t require you to compile the plug-in outside of Unity and re-import the result. If you plan to modify the plug-in at all in the future, or want to iterate over it often, use an Android Library. After you finish development for the plug-in, compile it into an Android Archive.
An Android Library is a directory with a specific structure that contains all the plug-in assets and the manifest.
When Unity creates the final Gradle project during the build or export process, it automatically includes all Android Library Projects in it and builds them together. Unity does this in the same way that Android Studio projects build when they have multiple subprojects.
An Android Archive (AAR) plug-in is a compiled version of an Android Library project that you can use as a dependency for an Android app module. The .aar file itself is a .zip archive that contains all of the compiled code, assets, and plug-in manifest. For more information on the structure of an AAR, see Anatomy of an AAR file.
If you need to add Assets to your Unity application that should be copied as they are into the output package, include the raw assets in an Android Library Project or AAR. To access these assets, call the getAssets Android API from your Java code.