architecture of an app with logic in the backend

[Asif Amin]

Hey, I am about to begin to build an app and I have choosen to develop the app in Flutter, and connected it to Firebase. However this is where I struggle to understand what Firebase does. My intial thought was to use Flutter -> node.js(host API and backend logic) -> mongoDB But I see that if I implement Firebase it would be easier for scalability, authentication, microservices and a lot more if I use Firebase, where would I connect node.js in the implementation to handle the logic of the app, as I do not want to add the logic in Flutter. Just want to confirm wether I am thinking right, I connect flutter directly to Firebase database (all authentication authorization is handled by Firebase) And from there I need to write a logic to whenever push a notification to user. Can I write the logic in Firebase?