Iterate through all users - firestore

I have the following (simplified) setup in firestore

    Collection     Document      FIELD     
    [USERID]   -   [STATUS]   -   date_of_birth: timestamp
                                  last _update: timestamp

    [USERID]   -   [STATUS]   -   date_of_birth: timestamp
                                  last _update: timestamp

    [USERID]   -   [STATUS]   -   date_of_birth: timestamp
                                  last _update: timestamp

    [USERID]   -   [STATUS]   -   date_of_birth: timestamp
                                  last _update: timestamp
...

I want to build a firebase function where, based on the date of birth, the last_update is not allowed to be older than one year (365 days). If last_update > 365 I want to send the user an notification email.

My problem is, how can I iterate through all the [UDERID]'s? What is the best way to do this? Is there a way to get all USERID's (more than 4000) in an array and iterate through them or is there another way?

I saw that there should be a way https://firebase.google.com/docs/auth/admin/manage-users#node.js but I have no idea how to implement this within google cloud functions.

Does someone has an idea/solution???

Thanks a lot!

Okay, I realized that the design of my database isn't very optimal. If anyone is interested in the solution, please find it below.

This is the index.js for functions:

const functions = require("firebase-functions");
const admin = require('firebase-admin')
const serviceAccount = require('./cerdentials.json')
admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: "https://test-data-eda4b.firebaseio.com"
});

const nodemailer = require("nodemailer");

let transporter = nodemailer.createTransport({
    service: 'gmail',
    // true for 465, false for other ports
    auth: {
        user: 'your email', // generated ethereal user
        pass: 'passowrd', // generated ethereal password
    },
});

const auth = admin.auth();
const db = admin.firestore();

exports.sendEmailToUsers = functions.pubsub.schedule('every 120 minutes').onRun(async (context) => {
    await fatchUsers("").then(res => {
        console.log(res);
    }).catch(e => { console.log(e) })
})

async function fatchUsers (token) {
    if (token != "") {
        return await auth.listUsers(4, token).then(async (usersList) => {
            usersList.users.forEach(user => {
                db.collection(user.uid).doc('status').get().then(async snapshot => {
                    if (snapshot.exists) {
                        if (snapshot.data().notifications_softskills_activated) {
                            console.log("send email to this user : " + user.email);
                            var Email_Res = await sendMail(user.email)
                            functions.logger.info(Email_Res)
                        }
                    }
                })
            })
            if (usersList.pageToken != undefined) {
                await fatchUsers(usersList.pageToken)
            } else {
                return true;
            }
        })
    } else {
        return await auth.listUsers(4).then(async (usersList) => {
            usersList.users.forEach(user => {
                db.collection(user.uid).doc('status').get().then(async snapshot => {
                    if (snapshot.exists) {
                        if (snapshot.data().notifications_softskills_activated) {
                            console.log("send email to this user : " + user.email);
                            var Email_Res = await sendMail(user.email)
                            functions.logger.info(Email_Res)
                        }
                    }
                })
            })
            if (usersList.pageToken != undefined) {
                await fatchUsers(usersList.pageToken)
            } else {
                return true;
            }
        })
    }
}

async function sendMail (email) {
    return await new Promise(async (resolve, reject) => {
        const mailOptions = {
            from: "testuser@gmail.com", // sender address
            to: email, // list of receivers
            subject: 'test mail', // Subject line
            html: '<h1>this is a test mail.</h1>'// plain text body
        };
        transporter.sendMail(mailOptions, function (err, info) {
            if (err) {
                reject(err);
            }
            else {
                resolve(info);
            }
        })
    })
}