Basically then, an aircraft manufacturer approaches specialist equipment suppliers with a description of their requirements and sees if they are currently producing anything suitable. If not, then would they be willing to design and supply something new in return for a commitment of a minimum number of orders or sole supplier status.
A modern commercial airliner is far too involved to be built by a single entity. Airbus and Boeing simply couldn't produce everything in house as it would be too expensive to develop the expertise to match outside suppliers who have been concentrating on a single area, such as landing gear or brakes for the last fifty years.