Каким образом можно реализовать механизм получения push-уведомлений от сервера без использования сервисов Google и Huawei?

To implement push notifications without using Google services (Firebase Cloud Messaging) or Huawei, you can use your own server and protocols, such as WebSocket or MQTT.

Options:

• WebSocket: establishes a persistent connection between the client and server, allowing the server to send notifications at any time.
• MQTT: a lightweight messaging protocol suitable for mobile devices.
• Polling: the client periodically checks the server for new messages (less efficient method).

Example of using WebSocket on Android with the OkHttp library:

val client = OkHttpClient()
val request = Request.Builder().url("wss://yourserver.com/socket").build()
val listener = object : WebSocketListener() {
    override fun onMessage(webSocket: WebSocket, text: String) {
        // Handling received notification
    }
}
val ws = client.newWebSocket(request, listener)

Thus, you can implement your own push notification mechanism without relying on third-party services.