It uses Parse API in Unity to send notification and read/write on Back4App database and fixes the problem of missing java classes, since Parse is outdated and has conflicts with recent versions of Unity and Firebase
you need to add your own google-services.json, provide your own app id, client id, dot net id, sender id, all taken from Firebase console and Back4App console
I explained more inforemation about the issue here https://stackoverflow.com/questions/56562370/how-to-configure-parse-sdk-client-to-receive-push-notification-in-unity-3d/75636343#75636343