If you have fundamental questions about coding in 6502, you can feel free to ask them here. I also recommend reading and trying https://skilldrick.github.io/easy6502/
In GameEngineData/demo.txt is all code used to generate the ROM. (All generated files, and all engine files).
If you want to read NES Maker's engine code, it starts in GameEngineData/MainASM.asm
All other code files the program uses are included from there. For instance, on the third line is
Code:
.include "Routines\System\Header.asm"
".include" basically means copy/paste that entire file in place of this line. So you could open that file to see what it contains. And that file may include other files.
The NES Maker program primarily generates data, not code. This means that checking it out would not have much context to learn from without also reading the code included that's meant to use that data to accomplish a task.
The data is primarily exported to GameEngineData/GameData (most things in that folder), GameEngineData\ScreenData (most things in that folder) GameEngineData\ScreenData2 (most things in that folder), GameEngineData\Sound\AllSongs_WithSFX.asm, GameEngineData\Routines\Variables\UserVariables.asm, GameEngineData\Routines\InitializationScripts\hudVarInits.asm, GameEngineData\Routines\System\HandleLoadHud.asm, GameEngineData\Graphics\Backgrounds (most things in that folder). There might be more?
I don't know of a way to view specifically only these files from the GUI itself, but I'm not sure how helpful it'd be. It would be less helpful for learning than reading the non exported code starting at MainASM.asm. But even then, I don't think reading that will be that helpful to learn from either unless you already know 6502 and NES programming.