In recently purchased Xiaomi Redmi Note 12 Pro with MIUI I see the message app isn’t available for many popular apps, those messages usually show when the android version is old but I have android 13 so I don’t understand why I see those errors so much in the play store.
Huaweii’s HarmonyOS is also based on Android, just like other manufacturer’s Android reskins.
They might’ve hard forked AOSP without incorporating newer changes and over time farther deviate from upstream AOSP, but at least three years ago they basically just replaced all mentions of Android with Harmony.
https://arstechnica.com/gadgets/2021/02/harmonyos-hands-on-huaweis-android-killer-is-just-android/