The demo HMI files can be found in the GitHub library repo
https://github.com/ScruffR/ITEADLIB_Nextion/tree/master/firmware/examples
There is a font creation tool in the Nextion Editor menu Tools - Font Generator
The firmware is created by the Nextion Editor and I usually use the SD card to update the display but you can use an FTDI cable too - but I find the SD update much quicker.