develop features in an existing mobile and web application

I am looking for a developer to work on adding functionality to my existing mobile and web application, which is primarily written in Java. The following are the specific requirements for this project:

Functionality:
- Implement a notification system for user activity
- Integrate a payment gateway for in-app purchases
- Develop a secure messaging system for users
- Implement a user rating system for services offered

Ideal skills and experience:
- Strong proficiency in Java and Android/iOS development
- Experience with integrating payment gateways and messaging systems
- Knowledge of secure coding practices and user authentication
- Familiarity with rating systems and push notifications

Task
develop subscription based model in mobile app and web
the processing fee of stripe and paypal should be charge to customer (end user)
A platform free of amount 1.5 USD should be charge to every user (end user)
integrate circle.com API for sending and receiving of USDT
integrate mirror-ai with the existing system and create a package in the app and web

Developmental languages used

IOS
Programming Language: Swift
Video Calling API: Vonage
Real-time Operations: Signal-R
Payment Gateways: Paypal and Stripe
ANDROID
Programming Language: Kotlin
Video Calling API: Vonage
Real-time Operations: Signal-R
Payment Gateways: Paypal and Stripe
WEB APP/ADMIN AREA/ REST API FOR MOBILE APPS
Programming Language: C#.net
Framework: ASP.Net Core 3.1
IDE: Microsoft Visual Studio 2022
Database: MSSQL Server version: 14
IDE Used for DB: SQL Server Management Studio 2017

If you have any questions or suggestions for improving the functionality, please feel free to share them. I am looking for a developer who can work efficiently and deliver high-quality results within a reasonable timeframe.