Alright, before diving into Firebase Firestore, we need to get Firebase into our project. For a web app, just pop these scripts into your HTML file:

<!-- Firebase App -->
<script src="https://www.gstatic.com/firebasejs/VERSION/firebase-app.js"></script>

<!-- Firebase Firestore -->
<script src="https://www.gstatic.com/firebasejs/VERSION/firebase-firestore.js"></script>

Don't forget to swap out VERSION with the specific version of Firebase you're using.

Next up, we need to initialize Firebase. Replace your-project-id, your-app-id, and your-api-key with your own Firebase project's details.

const firebaseConfig = {
  apiKey: "your-api-key",
  authDomain: "your-project-id.firebaseapp.com",
  projectId: "your-project-id",
  appId: "your-app-id",
};
// Initialize Firebase
firebase.initializeApp(firebaseConfig);

// Initialize Firestore
const db = firebase.firestore();

To add a new document to a collection and store some data, use this method:

let data = {
  name: 'John Doe',
  age: 34,
  children: [
    {name: 'Anna Doe', age: 7},
    {name: 'Sarah Doe', age: 5}
  ]
};

db.collection('users').doc('user1').set(data);

Here, users is the collection name, user1 is the document ID, and data is the object with the data you want to store.

To fetch the data stored in a document, you can use the get() function.

db.collection('users').doc('user1').get().then((doc) => {
  if (doc.exists) {
    console.log("Document data:", doc.data());
  } else {
    console.log("No such document!");
  }
}).catch((error) => {
  console.log("Error getting document:", error);
});

If the user1 document exists, its data will be logged in the console. Otherwise, you'll see "No such document!".

To access nested document values, you need to mention the full path to the nested value. Here's how you can get the data from the children array in the stored document.

db.collection('users').doc('user1').get().then((doc) => {
  if (doc.exists) {
    let childrenData = doc.data().children;
    childrenData.forEach(child => {
         console.log("Child's name: " + child.name);
         console.log("Child's age: " + child.age);
    });
  } else {
    console.log("No such document!");
  }
}).catch((error) => {
  console.log("Error getting document:", error);
});

This will log the name and age of each child in the nested 'children' array.