I'm sorry to ask you a basic question.

As I make games with Nani novel, '.GetUI' often appears.

I'm a little confused because I didn't see it when I was learning C# or Unity engine.

I searched through Google or the guide, but it doesn't come out anywhere.

Is it a function supported by Nani Nobel?

.GetUI() maybe a method under Naninovel.IUIManager ?
This method is in naninovel source code, not C# or Unity engine code

That's right, it's part of Naninovel's C# codebase.