<PackageReference Include="System.Configuration.ConfigurationManager" Version="10.0.3" />

System.Configuration.Internal.IInternalConfigSettingsFactory

public interface IInternalConfigSettingsFactory
Defines an interface used by the configuration system to set the ConfigurationSettings class.
void CompleteInit()

Indicates that initialization of the configuration system has completed.

void SetConfigurationSystem(IInternalConfigSystem internalConfigSystem, bool initComplete)

Provides hierarchical configuration settings and extensions specific to ASP.NET to the configuration system.