Not sure of the process

So today is the day to decide the software to use with my cnc router project. I'm not sure of the electro is pieces involved. I'll lay out what I think I know and you can correct me at will.
Working backwards, i get the motor/ driver interface. Where I go dumb is from the BOB to the computer. It seems the BOB is sometimes proprietary to the cam software. So a software decision needs to be made.
Another brain fog moment comes from the insertion of andrinos and Rpi. I'm not sure why this is there. Is it so you don't have to tie a computer proper to the machine? Why the extra processor? Is it just so you can use grbl or mach? What are my software (cam) options if I just want to port from a motherboard mounted in the box (with associated hardware). Would this mean I only need controller software on this puter? Which one software?
If I run a cad system like Catia (I have it already) and use the machining work bench to create cutter paths, what do I use to post process? Does the postP determine what controller software to use? Somebody point me to a link or explain this to me
Thanks for your time and attention.

The normal way of getting a 3D (or 2D) model into the machine controller is CAD>CAM>Post Processor>Machine Controller.

The post processor is dictated by the machine controller/software rather than the other way around.

Normally you would start out by installing your choice of machine control software/hardware then use the matching post processor for your favorite CAM software.

The machine control hardware/software could be anything from an Arduino to a crazy expensive industrial system. Your choice. Mach3/4 is popular in the hobby class, as is LinuxCNC, and many others.