React-Native – How is it possible that when I close an app like messenger I can still receive incoming call notifications from others

I am building an app like Messenger, everything works normally like sending messages, receiving messages, video calling. the technologies i use are react-native, reactJS, socket.io, nodeJS, react-native-webrtc

I wonder how can when I close an app like the messenger I can still receive incoming call notifications from other people. I’m studying firebase notification and react-native-background-timer but it doesn’t seem to work the way I want it to

Has anyone had a problem like this and have an answer, please let me know
Have a nice day guys <3