Two factor authentication (MFA) in ASP.NET Core application with Identity Server 4

I have already created login flow with Identity Server 4 in my .NET Core application, but it is customized. I mean, flow is not following the classes like UserManager / SignInManager as Identity Server default implementation for user management, even tables for user are also customized in the database.

Please note, my project does not have Microsoft.AspNetCore.Identity.IdentityUser as default identity.

Now I want to implement two factor authentication via Google Authenticator app in my application.

I have tried https://deblokt.com/2019/10/09/07-identityserver4-mfa-totp/ document, but did not have much success as login flow is not following legacy flow of Identity Server 4.

My questions are:

1. Can I still add Google Authenticator to achieve multi-factor authentication?
2. If yes, then how? Any suggestion or tutorial which I can refer to?
3. Please correct me if I misunderstood the concept

Thanks in advance.