index.ts

import * as functions from "firebase-functions" // The Cloud Functions for Firebase SDK to create Cloud Functions and set up triggers.
import { Firestore } from "@google-cloud/firestore"; // Cloud Firestore: Node.js Client
import * as admin from "firebase-admin"; // The Firebase Admin SDK to access Firestore.

admin.initializeApp();
const db = admin.firestore();
const adminUSA = new Firestore({projectId:"firestoreusa"}); // Firebase USA access account

const collections = ["bridges", "users", "tokens"];

collections.forEach((collection) => {
    exports["create_"+collection] = functions
        .region('europe-west1')
        .firestore
        .document(collection+'/{id}')
        .onCreate(onCreate)
    exports["update_"+collection] = functions
        .region('europe-west1')
        .firestore
        .document(collection+'/{id}')
        .onUpdate(onUpdate);
    exports["delete_"+collection] = functions
        .region('europe-west1')
        .firestore
        .document(collection+'/{id}')
        .onDelete(onDelete);

    exports["delete_"+collection] = functions
        .region('europe-west1')
        .firestore
        .document(collection+'/{id}')
        .onWrite(onWrite);

    })

async function onCreate(snap: functions.firestore.QueryDocumentSnapshot) {
    console.log('onCreate', snap.ref.path, snap.data());
    await adminUSA.doc(snap.ref.path).set(snap.data()); 
}

async function onUpdate({ before, after}: { before: functions.firestore.QueryDocumentSnapshot, after: functions.firestore.QueryDocumentSnapshot}) {
    console.log('upUser', before.ref.path, after.ref.path);
    if (before.ref.path === after.ref.path) {
        // same path, different values
        await adminUSA.doc(after.ref.path).set(after.data());
    } else {
        // moved
        await adminUSA.doc(before.ref.path).delete();
        await adminUSA.doc(after.ref.path).set(after.data());
    }
}

async function onDelete(snap: functions.firestore.QueryDocumentSnapshot) {
    console.log('onDelete', snap.ref);
    await adminUSA.doc(snap.ref.path).delete();
}

async function onWrite(change: functions.Change<functions.firestore.DocumentSnapshot>) {
    console.log('onWrite', change.before, change.after);
    // QUESTION how do Deletes look?
    // QUESTION how do Inserts look?
}

package.json

{
  "name": "firestore-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.ts",
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@google-cloud/firestore": "^5.0.2",
    "firebase-admin": "^10.2.0",
    "firebase-functions": "^3.21.2",
    "typescript": "^4.7.2"
  }
}

tsconfig.json

{
    "compilerOptions": {
        
    }
}