Me and my friends are building the same app on different platforms, but with a single RESTful API built with Node.js and MongoDB. If, for instance, a user changes something, like a setting, and the setting gets updated in the database, how can I make it so other users get notifications from their app? Like one user sends a friend request to me, and then I get a notification with the friend request. This applies to both Flutter and SwiftUI. It seems weird for me, because working with this API, it's only about requests and responses, and I don't have any idea how to make the API trigger notifications on the app. So please, tell me a way to do this, or something I can look into. Thank you.