WebApplicationBuilder is the mechanism for configuring the kestrel web server. Kestrel is the foundation of Asp.Net Core, there's no separating the two. But Kestrel is a totally modular system. If you configure your request pipeline with a single custom middleware then that is literally the only thing the server is running. If you use only minimal APIs then the middleware that handles their routing is the only thing running. Asp.Net Core has dozens of bells and whistles but none of them affect your app if they aren't part of your request pipeline.