Programming an Arduino CAN BUS board

Good evening, I could use some help.
I need a new control unit (implemented with Arduino) for my Audi TT 8N 1.8t due to modifications. However, I also need to read 2 data points from the CAN bus, and I would like to know the following:

-What does the signal look like between the CAN BUS board and the Arduino? I want to program it, but I can't do that if I don't know the signal type (analog? digital? etc.).

-What exactly does the CAN BUS board do, and how do I program it?

Thank you in advance!

It depends on which shield you want to use. Most often, the CAN modules are accessed via the SPI bus (e.g., with the MCP2515).