Jellyfin is a free software media system that manages, organizes, and streams personal media libraries (movies, TV shows, music, photos, books, live TV). The backend server is written in C# using ASP.NET Core and provides a REST API, WebSocket support, and a plugin system. It handles media metadata aggregation from multiple external providers (TMDB, MusicBrainz, AudioDB, etc.), transcoding via FFmpeg, user authentication and authorization, and real-time session synchronization (SyncPlay). The architecture is modular with separate packages for API controllers, database persistence (EF Core with SQLite), media file naming/parsing, metadata providers, and background scheduled tasks. Key entry points include the main server startup (Jellyfin.Server), API controllers (Jellyfin.Api), and the plugin system (MediaBrowser.Providers.Plugins). External dependencies include FFmpeg for transcoding, SQLite for the database, and various third-party metadata APIs. The codebase demonstrates strong engineering practices with extensive unit and integration tests, a custom event-driven architecture, and a well-defined authorization handler pattern for fine-grained access control. While there are some legacy namespaces and a large number of database migrations indicating schema evolution, the overall structure is clean and maintainable.