Bare React Native recipe

npm install @react-native-firebase/app @react-native-firebase/messaging @notifee/react-native react-native-device-info
cd ios && pod install

import { PermissionsAndroid, Platform } from "react-native";
import messaging from "@react-native-firebase/messaging";
import notifee, { AndroidImportance } from "@notifee/react-native";
import DeviceInfo from "react-native-device-info";

const REGISTER_URL = "https://app.nativenotify.com/api/universal/device/register";

async function postRegistration({ appId, appToken, subscriberId }, token) {
  const res = await fetch(REGISTER_URL, {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({
      appId,
      appToken,
      deviceId: await DeviceInfo.getUniqueId(), // IDFV on iOS, ANDROID_ID on Android
      platform: Platform.OS,
      subscriberId, // your logged-in user's id; omit for an anonymous device
      tokens: Platform.OS === "ios" ? { apnsToken: token } : { fcmToken: token },
    }),
  });
  if (res.status !== 201) {
    // 400/403 answer { error: { code, message } }; a wrong appId/appToken answers 401 as plain text.
    throw new Error(`Native Notify register failed (${res.status}): ${await res.text()}`);
  }
  return res.json(); // { ok, device, tokens }
}

export async function registerUniversalDevice(args) {
  // Without permission nothing is shown (tokens are issued either way).
  if (Platform.OS === "android" && Number(Platform.Version) >= 33) {
    await PermissionsAndroid.request(PermissionsAndroid.PERMISSIONS.POST_NOTIFICATIONS);
  } else if (Platform.OS === "ios") {
    await messaging().requestPermission();
  }

  // iOS: the raw APNs token (null until APNs has delivered it). Android: the FCM token.
  const token = Platform.OS === "ios" ? await messaging().getAPNSToken() : await messaging().getToken();
  if (!token) return null; // not ready yet — the next launch registers
  return postRegistration(args, token);
}

// FCM rotates Android tokens — register the new one.
export function watchUniversalToken(args) {
  return messaging().onTokenRefresh((fcmToken) => {
    if (Platform.OS !== "android") return;
    postRegistration(args, fcmToken).catch((err) => console.warn("Native Notify:", err));
  });
}

// Android pushes are FCM *data* messages: show them yourself (foreground and background).
export async function showNativeNotifyMessage(remoteMessage) {
  const data = remoteMessage.data || {};
  if (!data.title) return;
  const channelId = await notifee.createChannel({
    id: data.channelId || "default",
    name: "Notifications",
    importance: AndroidImportance.HIGH,
  });
  await notifee.displayNotification({
    title: data.title,
    body: data.message,
    data: { pushData: data.body || "{}" }, // your pushData JSON (+ nn_notification_id / nn_source)
    android: { channelId, pressAction: { id: "default" } },
  });
}

// index.js — before AppRegistry.registerComponent (the background handler must be set early)
import { Platform } from "react-native";
import messaging from "@react-native-firebase/messaging";
import { showNativeNotifyMessage } from "./universalPush";

if (Platform.OS === "android") {
  messaging().onMessage(showNativeNotifyMessage);
  messaging().setBackgroundMessageHandler(showNativeNotifyMessage);
}