The devices are sending MIDI over USB, so you need a USB host that communicates with the devices, power them and generates the sound. The host can be a PC, Synth or Mobile phone. To connect several devices simply use a USB hub. There are great programs free of charge you can try, like sunvox.
I thought that they were physically chained somehow. So a program like sunvox knows to take all the connected USB devices that identify as midi and mix them?
BTW, I have tons of electronic stuff lying around, like arduino nano, potentiometer, leds and others, but ofcourse none are compatible for the projects I choose… I am waiting for new ones, and I will start printing in the meantime.