You don't need any lib for it. The nextion display stores and loads the UI in an SD card, and communicates with the Arduino or esp by serial com. Depending on the control, it will pass the control I'd and value in the serial string.
The UI is composed using the Nextion UI editor, and really is easy to use.
I must confess, I haven't used mine in quite some time, so I think that I haven't kept any code from my tinkering...