How can I send a OneSignal push notification to a specific user on a new follow event in iOS?

I'm developing an iOS app using OneSignal for push notifications and Firebase as my backend. My goal is to send a push notification only to the user who gets a new follow. I have already set up OneSignal integration and my Firebase Firestore transaction successfully updates the followers and following counts, but I'm not sure how to trigger the notification for the targeted user.

I’ve implemented a function for handling the follow action and a notification service extension to process incoming notifications. My current code is as follows:

SWIFTCopied
static func follow(uid: String) async throws {
        guard let currentUid = Auth.auth().currentUser?.uid else { return }
        
        let userFollowingDoc = FirestoreConstants.FollowingCollection.document(currentUid).collection("user-following").document(uid)
        let userFollowersDoc = FirestoreConstants.FollowersCollection.document(uid).collection("user-followers").document(currentUid)
        
        try await Firestore.firestore().runTransaction { transaction, errorPointer in
            do {
                let userDoc = FirestoreConstants.UserCollection.document(uid)
                let currentUserDoc = FirestoreConstants.UserCollection.document(currentUid)
                
                transaction.updateData(["stats.followers": FieldValue.increment(Int64(1))], forDocument: userDoc)
                transaction.updateData(["stats.following": FieldValue.increment(Int64(1))], forDocument: currentUserDoc)
                
                transaction.setData([:], forDocument: userFollowingDoc)
                transaction.setData([:], forDocument: userFollowersDoc)
            } catch {
                errorPointer?.pointee = error as NSError
                return nil
            }
            return nil
        }
    }

import UserNotifications
import OneSignalExtension

class NotificationService: UNNotificationServiceExtension {
    
    var contentHandler: ((UNNotificationContent) -> Void)?
    var bestAttemptContent: UNMutableNotificationContent?
    var receivedRequest: UNNotificationRequest!
    
    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
        self.contentHandler = contentHandler
        self.receivedRequest = request
        bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)
        
        guard let bestAttemptContent = bestAttemptContent else { return }
        
        // Configure the notification content
        OneSignalExtension.didReceiveNotificationExtensionRequest(
            self.receivedRequest,
            with: bestAttemptContent,
            withContentHandler: contentHandler
        )
        
        // Add custom notification handling if needed
        if let userInfo = bestAttemptContent.userInfo as? [String: Any] {
            // Example: Add a custom category for follow notifications
            if let notificationType = userInfo["type"] as? String, notificationType == "follow" {
                bestAttemptContent.categoryIdentifier = "FOLLOW_CATEGORY"
            }
            
            // Example: Modify the notification body based on the type
            if let senderName = userInfo["senderName"] as? String {
                bestAttemptContent.body = "\(senderName) \(bestAttemptContent.body)"
            }
        }
        
        // Add a custom sound
        bestAttemptContent.sound = UNNotificationSound.default
    }
    
    override func serviceExtensionTimeWillExpire() {
        // Called just before the extension will be terminated by the system.
        guard let contentHandler = contentHandler,
              let bestAttemptContent = bestAttemptContent else { return }
        
        // Use this as an opportunity to deliver your "best attempt" at modified content
        OneSignalExtension.serviceExtensionTimeWillExpireRequest(
            self.receivedRequest,
            with: bestAttemptContent
        )
        
        contentHandler(bestAttemptContent)
    }
}

I’m looking for guidance on how to trigger a OneSignal push notification for the correct user immediately after they receive a new follow. Should I use OneSignal’s REST API directly, or integrate with Firebase Cloud Messaging via Cloud Functions?

Any help or example code to achieve this targeted push notification would be greatly appreciated.