为什么有些应用程序会在没有后台模式的情

I was using the Kijiji app where you can buy/sell things, and everytime someone replies to my ad I receive a notification on my screen. My understand is that unless you follow the background modes given by apple you can't do any notifications right? how is push notifications regularly handled by Apple, how does the app track in the background when someone has replied and then notifies me? I was using the Kijiji app where you can buy/se