Alright, let's get Firebase up and running in your app. First things first, you need to include the Firebase SDK in your project and initialize it with your configuration. Here's a quick example:

// Import the functions you need from the SDKs
import { initializeApp } from 'firebase/app';
import { getFirestore } from 'firebase/firestore';

// TODO: Replace with your Firebase project configuration
const firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "YOUR_AUTH_DOMAIN",
  projectId: "YOUR_PROJECT_ID",
  storageBucket: "YOUR_STORAGE_BUCKET",
  messagingSenderId: "YOUR_MESSAGING_SENDER_ID",
  appId: "YOUR_APP_ID"
};

// Initialize Firebase
const app = initializeApp(firebaseConfig);
const db = getFirestore(app);

Make sure your collections are set up correctly. Collection group queries are super handy when you have multiple subcollections with the same name in different documents. For instance:

/cities/{cityID}/landmarks/{landmarkID}
/towns/{townID}/landmarks/{landmarkID}

So, both cities and towns documents have landmarks subcollections that we want to query together.

Next, import the essential Firestore functions from the SDK:

import { collectionGroup, getDocs, query, where } from 'firebase/firestore';

Now, let's create a collection group query to fetch data from subcollections with the same name across different collections.

// Create a reference to the collection group
const landmarksRef = collectionGroup(db, 'landmarks');

// Create a query
const q = query(landmarksRef, where('type', '==', 'museum'));

Time to execute the query, grab the results, and handle the data as needed:

const querySnapshot = await getDocs(q);

querySnapshot.forEach((doc) => {
  console.log(`${doc.id} => ${doc.data()}`);
});

This will print all landmarks documents with a type value of 'museum' from all landmarks subcollections across the Firestore database.

Don't forget to handle potential errors by adding a try-catch block around the query execution:

try {
  const querySnapshot = await getDocs(q);
  querySnapshot.forEach((doc) => {
    console.log(`${doc.id} => ${doc.data()}`);
  });
} catch (error) {
  console.error("Error performing collection group query: ", error);
}

With the data in hand, you can now do whatever you need with it, like displaying it on the UI or processing it further.

This step-by-step guide outlines how to perform Firebase Firestore collection group queries. Make sure you follow Firestore’s best practices for the best performance and security.