One is a cross-platform React framework that unifies web, iOS, and Android development under a single codebase. It provides a file-system based router with support for SSR, SSG, SPA, and API routes, a middleware system, and a comprehensive set of native platform APIs (camera, clipboard, storage, motion, speech, etc.) implemented via Nitro hybrid objects bridging JavaScript to Swift/Kotlin/C++. The framework includes a custom Vite plugin for web builds and a Metro-compatible build pipeline for native targets, along with a CLI for project scaffolding, dev server, and deployment. The monorepo also contains a documentation site (onestack.dev), example apps, and extensive test suites covering routing, loaders, native features, and cross-platform compatibility. The architecture emphasizes platform-specific file extensions (.native.tsx, .ios.tsx, .android.tsx) and a layout system with Slot components for nested composition.