In the end of Instant sync and save it says:

the method will want to pass in a Notification object with more information

Thus

func save(_ note: Notification) {
    dataController.save()
}

for

.onReceive(NotificationCenter.default.publisher(for: UIApplication.willResignActiveNotification)  , perform: save)

If I look into onReceive syntax:

func onReceive<P>(_ publisher: P, perform action: @escaping (P.Output) -> Void) -> some View where P : Publisher, P.Failure == Never

How did we know Type of Notification equals to Type of Publish.Output ?

If you go through the apple developer documentation for the

NotificationCenter.default.publisher(for: method Link

you will see that it returns a NotificationCenter.Publisher Link

and Notification is a typealias for this type Link