You need to looks at the buffers that go back and forth between the Nextion and the Raduino. Quite frankly, I never really figured them out because there is limited comment in the code. So not going to be a lot of help here.
There are some basic definitions in the code (this is a command, this should be the last buffer sent, etc.) And then you are going to see how they are uninterpreted on the Nextion side. (you will need to download the Nextion editor) to see how the buffers are interpreted there.
Much different than the typical interaction between Nextion and a Auduino script.
Sorry, cannot be more help.