Используйте admin.firestore()
как например:
// The Cloud Functions for Firebase SDK to create Cloud Functions and setup triggers.
const functions = requi re('firebase-functions');
// The Firebase Admin SDK
const admin = require('firebase-admin');
admin.initializeApp();
//Write a function that uses Firestore, e.g.:
exports.anHttpCloudFunction = functions.https.onRequest((req, res) => {
admin.firestore().collection('XYZ').doc('ABC').set(....)
.then(() => {
res.send({ msg: 'Success' });
})
.catch(err => {....})
});