The system-arm64-ab.img.xz file often comes from these community sources, especially for Android 12, which is less common in official Google GSIs after Android 13's release.
repo init -u https://android.googlesource.com/platform/manifest -b android-12.0.0_r45 repo sync source build/envsetup.sh lunch aosp_arm64_ab-userdebug make -j$(nproc) systemimage system-arm64-ab.img.xz android 12
It contains the core Android OS—the framework, system apps, and UI. The system-arm64-ab