ASP.NET Core was a rewrite. None of the previous .NET Framework or HttpListener HTTP classes or Abstractions are available in .NET Core which were rewritten to use newer HTTP Abstractions from scratch.
> The main differences are in startup configuration
It's definitely not the main difference, it may be what's immediately visible as a dev using it, but the entire Web Framework your App depends on is new. It's not a fork of an existing code base, it's a completely different one with the goal to retain the same development MVC & Web API model to preserve knowledge reuse & ease porting efforts.
The compatibility layers are just that, retrofitted to preserve compatibility & ease porting which is able to work through impl-free Reference assemblies with APIs they both share. Of course none of ASP.NET Framework System.Web is in .NET Standard, because it was rewritten.
> The main differences are in startup configuration
It's definitely not the main difference, it may be what's immediately visible as a dev using it, but the entire Web Framework your App depends on is new. It's not a fork of an existing code base, it's a completely different one with the goal to retain the same development MVC & Web API model to preserve knowledge reuse & ease porting efforts.
The compatibility layers are just that, retrofitted to preserve compatibility & ease porting which is able to work through impl-free Reference assemblies with APIs they both share. Of course none of ASP.NET Framework System.Web is in .NET Standard, because it was rewritten.
The OSS code base for ASP.NET Core is at https://github.com/aspnet whilst you can view source code of .NET Framework (which isn't OSS) is at: http://sourceof.net