A custom ROM is a complete, aftermarket replacement for your phone's Android operating system. For your Xperia 5 III, moving to a custom ROM is a way to:
Download the latest Android SDK Platform-Tools (adb and fastboot).
In recovery, select or Advanced > ADB Sideload .
Most Japanese carrier models (e.g., Docomo, Softbank) are permanently locked and cannot be easily unlocked without third-party paid services. 2. Recommended Custom ROMs The development community for the Xperia 5 III Go to product viewer dialog for this item. is concentrated on a few high-quality options: