access firestore from firebase-functions using web version 9 interface (modular)

I have to access a firestore database within firebase functions. I use the following code to get a firestore instance:

import { initializeApp } from "firebase-admin/app"
initializeApp();

import { getFirestore } from 'firebase-admin/firestore'
const db = getFirestore();

This instance can be used to access the database

db.collection('test').add( { anything: 123 } )
db.collection('test').doc('test123').set( { afield: 1234 } )
db.collection('test').doc('test123').get()

According to https://firebase.google.com/docs/firestore/query-data/queries#web-version-8 this is "Web version 8 (namespaced)" interface for accessing firestore databases.

How can I use the "Web version 9 (modular)" interface? The version 9 example from the same page:

import { collection, doc, setDoc } from "firebase/firestore"; 
const citiesRef = collection(db, "cities");

results in the following error:

FirebaseError: Expected first argument to collection() to be a CollectionReference, a DocumentReference or FirebaseFirestore

Is there a way to query firestore with admin privileges using modular interface?

Thank You!