This is not built yet. Nothing on this page describes something you can switch on today.
The idea is the third obvious destination for a custom notification: instead of a Discord message or a webhook, the match arrives as a push notification on your phone, through the mobile app you are already signed into.
Two things are worth separating, because they are easy to confuse:
In-app notifications already exist. Friend requests, event results and the rest appear under the bell on the website and in the mobile app today, in real time. Those are not push notifications; they arrive while you have the app open.
Push is the part that is missing. Reaching a phone whose screen is off needs device registration and a delivery service behind it, and neither is wired up yet.
Until then, a Discord notification is the closest thing, since the Discord app pushes to your phone for you.
This page will be filled in when the channel ships, and no earlier.