Android Question Push notification without using Firebase

Telegram has stated in its FAQ section that:
We currently have two types of notifications on Android: Google's Messaging cloud (GCM) and our own custom notification service that is independent of Google. Note that Google Notifications (GCM) may not work on some Android devices. Telegram notification service is reliable, but requires more battery resources. This consumption is not high and should not be, so please report any heavy battery consumption immediately.
I blocked FCM's IP on my network to make sure this works properly. Push notifications of all applications stopped working (for example, Instagram direct notification did not work), but I was surprised when I tried Telegram. Even though I had not opened Telegram for several hours, the chat notifications worked for me without any problems. (I was using Android 11)
What is more interesting is that Telegram does not ask for any permission for this.

How can I do this in b4a and implement a personal service for myself?

PS: Although I know that using Firebase is easier and better, I created this thread just to increase my knowledge and because I am excited about learning new things.