Where is the other ChatGPT Interface Hidden?

Does anyone know where the other interface for ChatGPT is located? Aka the one where you have 'System' and 'User' on the right.

Aka, this interface: https://github.com/dair-ai/Prompt-Engineering-Guide/blob/main/img/chatgpt-1.png

I have a Plus account, not sure if that's required, or maybe this is an internal interface not available to the public?