Steps I Followed and which worked like a Charm were :

Setup your Firebase:

• Same package name as your game
• put team ID in project setting of firebase
• upload APN Authentication file and its key ID in firebase project setting.
• Dload and place GoogleService-Info.plist file from Firebase to root of Assets folder of your project

Setup Unity:

• Add your TeamID from appstore connect to the field available under project setting of your Unity project

• also check true auto init next to teamID field in project setting

use this code to init Firebase

private void Start()
{
    Firebase.FirebaseApp.CheckAndFixDependenciesAsync().ContinueWith(task =>
    {
        var dependencyStatus = task.Result;
        if (dependencyStatus == Firebase.DependencyStatus.Available)
        {
            Debug.Log("Firebase Init");
            Firebase.Messaging.FirebaseMessaging.TokenReceived += OnTokenReceived;
            Firebase.Messaging.FirebaseMessaging.MessageReceived += OnMessageReceived;
        }
        else
        {
            UnityEngine.Debug.LogError(System.String.Format(
              "Could not resolve all Firebase dependencies: {0}", dependencyStatus));
        }
    });
    
}

#region FIREBASE MESSAGING

public void OnTokenReceived(object sender, Firebase.Messaging.TokenReceivedEventArgs token)
{
    UnityEngine.Debug.Log("Received Registration Token: " + token.Token);
}

public void OnMessageReceived(object sender, Firebase.Messaging.MessageReceivedEventArgs e)
{
    UnityEngine.Debug.Log("Received a new message from: " + e.Message.From);
}

#endregion

Make build after this

Setup Xcode :

• From capabilities windows add "Push Notification" and "Background Modes", in Background modes check remote notification and your almost all done..