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.
MIUI is sort of a homebrew OS broadly based on Android. Some things work, others don’t. I wouldn’t be surprised if Xiaomi and OPPO will eventually go the way of Huaweii and develop their own custom OS to lock users into their ecosystem. It’s a China thing.
And while I’m a big fan of Xiaomi products in general, I wouldn’t use any of their phones without flashing the EU ROM first to ensure compatibility with free market tech. If you’re willing to give that a try, the folks at xiaomi.eu are working hard to maintain compatibility.
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/