Comment by Kwpolska

C# can be used in both .NET Framework and modern .NET (ex-core). In fact, it is possible for a C# project to target both .NET Framework and .NET with the exact same code, or to target to .NET Standard, where the same .DLL file can be loaded by both. Since the old Framework is in maintenance mode, some modern language features will not work there, but you can still be productive with the old framework.

Dynamic is largely unnecessary, and it was unnecessary even when it was introduced.

ASP and ASP.NET are completely unrelated. ASP was designed to allow dynamic webpages to be written in VBScript (like CGI). This is not something you want to do in modern languages.