Cross-Platform App Development

I'm seeking an experienced app developer to design and build an app for me using Xcode. The app will be published on the App Store and should be compatible with iOS, iPadOS, and macOS.

Core functionalities required:
- User authentication
- Push notifications
- In-app purchases

Ideal Skills and Experience:
- Proficiency in Xcode and Swift
- Experience with app development for iOS, iPadOS, and macOS
- Strong background in implementing user authentication, push notifications, and in-app purchase systems
- Prior experience with App Store deployment

Please provide examples of previous work and a timeline for project completion.