USB-USB data protocol ?

Hello,
I am switching to the Advanced section in hope to find a solution to this apparently easy problem.
I am going to have around 100 RaspberyPi each one connected to a PC.
The Pi will handle hardware input/output (lights, speakers, fingerprint readers, QR readers, etc.) and the PC will display some interactive graphics.

I need an easy to maintain and reliably way to communicate between the Pi and the PC. I already did it once using the ethernet cable but it was really complicated to maintain (creating another network, replacing faulty devices, etc.) so this time I’ve been thinking to use USB.
I thought sending/receiving data via USB - like in a serial way - between two machines was a common enough/easy technique but alas I can’t find anything about it (protocol, example, requirements, …).

Has anyone any idea/advice on how to realise this ?

All help welcome.

Thank you.