Cross-Platform High-Availability App Development

I need a cross-platform app developed for Android, iOS, and Web. The app should handle millions of simultaneous logins and be highly available.

Key Requirements:
- Primary function: To be discussed in messages
- User authentication: Email and Password
- Payment integration: Accept payments globally via PayPal, Payoneer

Ideal Skills and Experience:
- Expertise in building scalable, high-availability applications
- Experience in cross-platform app development
- Proficient in integrating diverse payment systems
- Strong knowledge of secure authentication methods