Jitsi is open source, so you can look at the exact mechanics of their billing counter to answer your question:

It's all orchestrated from here: https://github.com/jitsi/jitsi-meet/blob/master/react/featur...

Which generates the MAU ID via:

react/features/billing-counter/functions.js:80

/* * Returns the stored billing id (or generates a new one if none is present). * * @returns {string} */

export function getBillingId() {

    let billingId = jitsiLocalStorage.getItem(BILLING_ID);

    if (!billingId) {
        billingId = uuid.v4();
        jitsiLocalStorage.setItem(BILLING_ID, billingId);
    }

    return billingId;
}