SdkExtensions is a module that decides the extension SDK level of the device, and provides APIs for applications to query the extension SDK level.
The module is packaged in an apex, com.android.sdkext, and has two components:
bin/derive_sdk: Native binary that runs early in the device boot process and reads metadata of other modules, to set system properties relating to the extension SDK (for instance build.version.extensions.r).javalib/framework-sdkextension.jar: This is a jar on the bootclasspath that exposes APIs to applications to query the extension SDK level.derive_sdk is a program that reads metadata stored in other apex modules, in the form of binary protobuf files in subpath etc/sdkinfo.binarypb inside each apex. The structure of this protobuf can be seen here. The exact steps for converting a set of metadata files to actual extension versions is likely to change over time, and should not be depended upon.
The module exposes a java class SdkExtensions in the package android.os.ext. The method getExtensionVersion(int) can be used to read the version of a particular sdk extension, e.g. getExtensionVersion(Build.VERSION_CODES.R).