The cable carries 24V power, the bi-directional CAN bus (CAN_H/CAN_L), and four lines which are connected to GPIO pins of the MCU in the controller, and which I seem to understand can have different functions depending on the module attached. For the linear modules they carry the stepper signals, for laser the PWM signal, for heated bed the thermistor etc.
I guess you have two options: You can emulate an existing toolhead (e.g. the 3DP head) and “abuse” certain GCode commands and the printer’s reactions for your own purpose, e.g. map the Extruder control to your syringe and the Fan control to your UV diode. Advantages: No need to change the firmware of the printer, and you may even use a standard slicer to generate your toolpaths. Disadvantages: You are restricted to how Snapmaker implemented the GCode interpretation.
Option 2: You implement your own module into the firmware. The firmware is open source and you can modify it - other users have done so, you’ll find this in the forum, e.g. Skreelink’s post: Full Lightburn Control Guide. Advantage: You have full control over GCode interpretation, and can even remap cable pin functions. Disadvantage: You fork from the official firmware, and it is a learning curve.
For Option 1: Other people (Ronin) have created emulated toolheads - see e.g. here: https://www.youtube.com/watch?v=1Shsjm7y94k and here: DIY Enclosure controls by Ronin - found on YouTube