A file with .apks extension is an archive file that is a collection of Android package APK files. Each individual APK file in the APKS file is generated keeping in view various aspects of the devices. These include architecture, language, screen density, and other device features. APKS files are generated by bundletool. It is used to build Android App Bundle and convert app bundle into difererent APKs for deployment on devices.
APKS File Format - More Information
APKS files are saved as compressed files using ZIP file format.
How to generate an APKS file?
When the Android App Bundle (AAB) is ready, its behaviour on Google Play store can be tested for deployment to a device. APKS files can be generated, for this purpose, from AAB files and installed on test devices using the Google’s Android bundletool. It provides command line tools to create the APKS archive file from APKs using the following command.