This specific "all [0]" variant is a universal build designed for standard smartphones and tablets, unlike variants specifically for Wear OS or Android TV.

: Automatically keeps your installed applications up to date to ensure security and stability. Why Users Look for Version 9.9.21 Legacy Device Support

Included variant support for Android TV, Wear OS (formerly Android Wear), and Android Automotive. How to Install the 9.9.21 APK

Version 9.9.21 is highly compatible with older Android versions (Lollipop and Marshmallow). Modern Play Store versions often require newer Android System WebViews or Google Play Services versions, which older devices cannot support. Therefore, v9.9.21 acts as a functional "last resort" for legacy devices.