Main Computer (MC)

The computer or system on chip that orchestrates all actions. Captain of the ship. Maestro of the choir. Usually the MC does the work of deciding when to command other systems and tell them what to do, decide what to do, and file system management.

An entire OS isn't really necessary here, but with a Raspberry Pi Zero and Raspberry Pi 4B+ on hand there's not really an advantage to running a system without one. The processors are fast enough to easily have headroom after dealing with OS level processes, and having an OS opens up the kinds of software that can run onboard.