Threema Android is a secure messaging application with end-to-end encryption using NaCl/libsodium. It features a modular architecture following clean architecture principles with separation into app, domain, data, and common layers. The app uses MVVM with ViewModels and StateFlow/LiveData, dependency injection via Koin, and Kotlin coroutines for asynchronous operations. Key functionality includes text/media messaging, voice/video calls, group chats, polls, contact management, backup/restore, multi-device synchronization, and availability status indicators. The application supports multiple build variants for different distribution channels (Google Play, Huawei AppGallery, F-Droid, Threema Shop) and includes enterprise features for business users. It relies on SQLCipher for encrypted local storage, implements custom CSP/D2M protocols for server communication, and provides web client functionality for desktop access.