Hi Franco and welcome to the forums.
What do you mean by "visualizing in Arduino"? The serial port connection is already used by Bonsai to communicate data. A good way to visualize the exact state of the digital ports is to connect an LED or plug it into an oscilloscope.
Also, what do you mean by "harness the digital data that Bonsai sends to Arduino in a sort of buffer"? Do you mean storing the data? It would help if you can provide the concrete example of what you are trying to do and why, so we can better contextualize the questions and answers. If this is not specifically about the serial port interface, please consider opening another question.
Hope this helps.