How does OpenIdConnectEvents get overrided?

[slowjams]

below is the source code:
https://source.dot.net/#Microsoft.AspNetCore.Authentication.OpenIdConnect/OpenIdConnectHandler.cs,1288

where await Events.AuthorizationCodeReceived(context); must exchange auth code with access code in the backchannel, but according to the source code of OpenIdConnectEvents:

https://source.dot.net/#Microsoft.AspNetCore.Authentication.OpenIdConnect/Events/OpenIdConnectEvents.cs

public class OpenIdConnectEvents : RemoteAuthenticationEvents
{
    public Func<AuthorizationCodeReceivedContext, Task> OnAuthorizationCodeReceived { get; set; } = 
        context => Task.CompletedTask;
    // ...
}

it does nothing but just returns Task.CompletedTask so this OnAuthorizationCodeReceived event must be override somewhere, but I couldn't see where it is getting override?