How do you create a subcollection in Firebase Firestore?

Question

I'm having troulbe figuring out how to add a subcollection to a document in my React App. I'm also getting the error that db.collection() is not a function. I'm trying to add the subcollection in the registerWithEmailAndPassword function. The Firebase Firestore documentation does not specify how to create a subcollection. Any help would be greatly appreciated thank you. Firebase config has been ommited to protect my API key.

import { initializeApp } from "firebase/app";
import {
  GoogleAuthProvider,
  getAuth,
  signInWithPopup,
  signInWithEmailAndPassword,
  createUserWithEmailAndPassword,
  sendPasswordResetEmail,
  signOut,
} from "firebase/auth";
import {
  getFirestore,
  query,
  getDocs,
  collection,
  where,
  addDoc,
  Firestore,
  doc,
  DocumentReference,
  setDoc
} from "firebase/firestore";

// TODO: Add SDKs for Firebase products that you want to use

// https://firebase.google.com/docs/web/setup#available-libraries

// Your web app's Firebase configuration

// For Firebase JS SDK v7.20.0 and later, measurementId is optional

// Initialize Firebase

const app = initializeApp(firebaseConfig);
const auth = getAuth(app);
const db = getFirestore(app);

const googleProvider = new GoogleAuthProvider();

const signInWithGoogle = async () => {
  try {
    const res = await signInWithPopup(auth, googleProvider);
    const user = res.user;
    const q = query(collection(db, "users"), where("uid", "==", user.uid));
    const docs = await getDocs(q);
    if (docs.docs.length === 0) {
      await addDoc(collection(db, "users"), {
        uid: user.uid,
        name: user.displayName,
        authProvider: "google",
        email: user.email,
      });
    }
  } catch (err) {
    console.error(err);
    alert(err.message);
  }
};

const logInWithEmailAndPassword = async (email, password) => {
  try {
    await signInWithEmailAndPassword(auth, email, password);
  } catch (err) {
    console.error(err);
    alert(err.message);
  }
};

const registerWithEmailAndPassword = async (name, email, password) => {
  try {
    const res = await createUserWithEmailAndPassword(auth, email, password);
    const user = res.user;
    const userDoc = await addDoc(collection(db, "users"), {
      uid: user.uid,
      name,
      authProvider: "local",
      email,
    });
    db.collection("users").doc(userDoc.id).collection("test");


  } catch (err) {
    console.error(err);
    alert(err.message);
  }
};

const sendPasswordReset = async (email) => {
  try {
    await sendPasswordResetEmail(auth, email);
    alert("Password reset link sent!");
  } catch (err) {
    console.error(err);
    alert(err.message);
  }
};

const logout = () => {
  signOut(auth);
};

export {
  auth,
  db,
  signInWithGoogle,
  logInWithEmailAndPassword,
  registerWithEmailAndPassword,
  sendPasswordReset,
  logout,
};