Typically disciplines cover Electrical, Industrial Electronics (PLC, Servo, Stepper, integration), Mechanics, Pneumatics, Hydraulics to name a few.
Even for someone that does this for a living, any new project can be a challenge.
This is often aggravated in the DIY world to some degree due to every aspect of a machine being put together piecemeal and items are sourced from many different suppliers and then have to be married together.
This makes it that much harder to describe a defined route or road map for a beginner to take.
This is why most questions have to be answered on a case by case basis.
That would be my consensus anyway.
The CNCzone is one big knowledge base