You are way ahead of me.
I too am trying to assemble the kit with the 3.5" Nextion display and the kit is full of mysteries.
I can't find the exact code for the 3.5" display. The published code doesn't fill the display.
Not sure yet about the right code for the Nano, still looking for the *.ino file for this.
Lots of different files, different generations, but the guide is fuzzy.
No clue how to install the audio amp or the AGC board.
I've got one uBitx working in the smaller case with a version 4 board. That was
a trial too, but not so bad.
Seems like clear instructions are promised in the future, but for now we are on our own.