Universal containers (UC) has built a custom based Two-factorAuthentication (2fa) system for their existing on-premise...

 Using custom login flows to connect to the existing custom 2fa system for use in salesforce is the recommended solution because it allows you to leverage your existing 2fa infrastructure and provide a consistent user experience across your applications. Custom login flows let you customize the authentication process by adding extra screens or logic before or after the standard login1. You can use Apex code to call your custom 2fa system and verify the user’s identity2. This option also gives you moreflexibility and control over the2fa process than using native 2fa or an app exchange app3. References: 1: Customize User Authentication with Login Flows 2: Custom Login Flow Examples 3: Salesforce Multi-Factor Authentication