I have seen everywhere how to send FCM(firebase cloud messaging) notifications from Arduino through different libraries.
However, I am wondering if there is a way to make Arduino subscribe to a notification topic and handle incoming FCM.
The reason i want this is because my mobile app has the ability to change/update certain data(flags) in my realtime database, but it's not that often, so i dont want my Arduino to keep looking for changes in the firebase realtime database.. instead, i want it to just subscribe for notifications and receive an FCM notification whenever anything has changed on the app level.
If an Android device (any device) doesn't have Play services installed, then it can't receive FCM messages. See the documentation on what's required for various Firebase products - FCM absolutely requires Play services.
Google typically requires that device manufacturers observe certain requirements for a device that wants Play services preloaded. If it's not a consumer device, it almost certainly is not installed. Perhaps you can try to install it yourself - start with a web search on that.