Some of the information on the AssemblyBuilder class page: https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Reflection.Emit/AssemblyBuilder.xml is describing the behavior of this API on .NET Framework not on .NET Core/.NET 5.

For example:

• All mentions of "saving the assembly" should be removed, the save functionality is not available in .NET Core

• To get an AssemblyBuilder object, use the AppDomain.DefineDynamicAssembly method.

  The AppDomain.DefineDynamicAssembly doesn't exist in .NET Core. The way to create the builder object is different now

• Basically any mention of AppDomain are stale as well

• AssemblyBuilderAccess enum has different values in .NET Core - the document doesn't reflect that change