By default .NET Core Apps are served from your App’s configured WebRoot which you can customize in your WebHostBuilder using UseWebRoot(). If you’re not using .NET Core you can change the physical directory with Config.WebHostPhysicalPath.
You can also add any number of cascading Virtual File Sources by adding registering them in InsertVirtualFileSources for VFS providers you want to have higher priority than the default VFS providers or AddVirtualFileSources for VFS providers with lower priority.