Answer
Kestrel is the built-in, cross-platform, high-performance web server for ASP.NET Core.
It is optimized for asynchronous I/O, low memory overhead, and extremely fast request handling, making it suitable for both self-hosted scenarios and running behind reverse proxies such as Nginx, Apache, or IIS.
ASP.NET Core uses Kestrel because it delivers excellent throughput, works on all supported platforms, integrates deeply with the middleware pipeline, and gives developers full control over how requests are processed.