How does Talk communicate to app?

My next cloud is an intranet and not open to the internet. Could someone tell my how my wife’s phone notifies her when I send her a message in talk while she is not connected to the network?

She receives a notification, but when she goes to the app as expected there is no message because she isn’t connected to the server. How is she getting a notification though…

Through a Push Service, which is operated by Nextcloud GmbH.

Your Nextcloud -> Push Service -> iOS/Android

Thank you!