Android Notifications Work over the Internet?

I don’t understand something.
I recently received a push notification from Home Assistant even though I wasn’t home.

How is that even possible? I can only reach my HA server over an OpenVPN tunnel.
How can I get android notification when I’m not in my local network?

Are you using Nabu casa?
Tell us more details about your system, it’s impossible to answer you at this stage.

I think I found it.
HA uses Google Firebase.