Каким образом можно реализовать отправку push-уведомлений пользователю?

To implement push notifications in iOS, the Apple Push Notification service (APNs) is used. Main steps:

1. Register the app in the Apple Developer Account and obtain push notification certificates.
2. Request user permission to receive notifications in the app using UNUserNotificationCenter.
3. Register the device with APNs and get a device token.
4. Send the device token to your server.
5. The server sends push notifications via APNs using the device token.

Example of requesting permission and registering the token:

import UserNotifications

UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { granted, error in
    if granted {
        DispatchQueue.main.async {
            UIApplication.shared.registerForRemoteNotifications()
        }
    }
}

// In AppDelegate:
func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    let tokenParts = deviceToken.map { data in String(format: "%02.2hhx", data) }
    let token = tokenParts.joined()
    print("Device Token: \(token)")
    // Send token to server
}

Thus, push notifications are sent by the server through APNs, and the app receives and displays them to the user.