At one time control units were ad-hoc logic, and they were difficult to design. Now they are designed as a microprogram that is stored in a control store. Words of the microprogram are selected by a sequencer and the bits from those words directly control the different parts of the computer, including the registers, arithmetic and logic unit, instruction register, bus, input/output and computer storage.
In modern computers, each of these subsystems may have its own subsidiary controller, but the control unit is the top dog that controls the computer overall.