Thanks, Carl! As I understand, they took the pinout and wrote the modified code for the processor of the printer, right?

That's about it for Repetier yes.

Howdy, people. I want to ask how Repetier Hack is developed?
If there are real developers, explain me as to a Python coder.
Is the firmware being reverse engineered to get the code?

Not sure what you mean by code.

In the case of Repetier all that was really done was the pin map redone to match that of the processor on the XYZ board. If you have the pinout of the processor you can remap the firmware as long as it runs on the processor in question.

For the Smaller da vincis they just  snooped the communication between the RFID chip and the processor and was able to create apps that will emulate the data.

There is a lot more to it but that is the process in a nutshell.