The design process starts with a requirement but there are optional approaches to a solution. The engineer can design and build the power system from discrete components; requiring at a minimum a micro-controller, some ADCs and DACS with the appropriate resolution and accuracy. Perhaps add some digital trim pots, several PWM controllers. If the requirement is for better than 1% accuracy, it is likely to need 0.1% precision resistors with good quality capacitors for the compensation network. It will also be necessary to allocate design time, a budget, test time, software development for the micro, board space and, if it is like most projects, more time and money than was originally requested to make it work properly.