How to send push notifications with Firebase?

public class MyFirebaseMessagingService extends FirebaseMessagingService {

@Override
public void onMessageReceived(RemoteMessage remoteMessage) {
    // Handle FCM messages here.
    // Not getting messages here? See why this may be: https://firebase.google.com/docs/cloud-messaging/android/receive

    if (remoteMessage.getNotification() != null) {
        // Get the message and handle it
        String messageBody = remoteMessage.getNotification().getBody();
        sendNotification(messageBody);
    }
}

private void sendNotification(String messageBody) {
    // Create and display the notification
    NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this, "default\_channel")
            .setSmallIcon(R.drawable.ic\_notification)
            .setContentTitle("FCM Message")
            .setContentText(messageBody)
            .setAutoCancel(true);

    NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION\_SERVICE);
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel channel = new NotificationChannel("default_channel", "Default Channel", NotificationManager.IMPORTANCE_DEFAULT);
        notificationManager.createNotificationChannel(channel);
    }

    notificationManager.notify(0, notificationBuilder.build());
}

import Firebase
import UserNotifications

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterDelegate, MessagingDelegate {

func application(\_ application: UIApplication,
                 didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Configure Firebase
    FirebaseApp.configure()

    // Set UNUserNotificationCenterDelegate
    UNUserNotificationCenter.current().delegate = self

    // Request notification permissions
    UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .badge, .sound]) { granted, error in
        // Handle the authorization request response
    }

    application.registerForRemoteNotifications()

    // Set the Messaging delegate
    Messaging.messaging().delegate = self

    return true
}

// Receive and handle FCM messages
func application(\_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any]) {
    // Handle the message
}

// Handle notification settings
func userNotificationCenter(\_ center: UNUserNotificationCenter,
                            openSettingsFor notification: UNNotification?) {
    // Handle the settings
}

// Handle FCM token refresh
func messaging(\_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String?) {
    print("Firebase registration token: (String(describing: fcmToken))")
}

// Initialize Firebase
const firebaseConfig = {
// Your Firebase config here
};
firebase.initializeApp(firebaseConfig);

const messaging = firebase.messaging();

messaging.getToken({ vapidKey: 'YOUR_PUBLIC_VAPID_KEY' }).then((currentToken) => {
if (currentToken) {
// Send token to server and display notification UI
console.log('Token:', currentToken);
} else {
console.log('No registration token available. Request permission to generate one.');
}
}).catch((err) => {
console.error('An error occurred while retrieving token. ', err);
});