This is just a proof of principle and by far not the simplest solution for this job.
C allows for directly addressing up to 127 "slave" boards. Here the board on the right side in the video is the "master" (which initiates all transfers), the left board is the slave (waiting for data). I2
C needs 2 signal lines and the usual power lines (+, -, SDA, SCL). As it is a bus, all devices are connected to it in parallel.