Telegram uses Google services like Signal for notifications - telegra.ph/Notifications-FIX its the first point under the Android section
core.telegram.org/api/push-updates these are the docs for building your own Telegram app, specifically the push notifications section and again it mentions using APNS for iOS or FCM for Android but they also offer Simple push which would work with Unifiedpush and would be one way to bypass FCM but I don’t know if they offer that in their official app or if there are any other Telegram apps that have implemented it