This page describes Electronics Development - the product design section of Research & Development. R & D is a bigger field than just this; it encompasses developing new techniques, totally new technologies, - pushing the boundaries. What we cover here is what is needed to use electronics to enable a new product to be built.
The majority of companies designing new Electronic products will go through a "Circuit - PCB - Prototype - Test" cycle that is described in detail on the webpage: The Electronics design method. This method does cost a reasonable amount of money, but is really quite versatile, and yields a production ready board design.
Not every new product starts off this way.
A budget constrained inventor might instead choose to make up the circuit with individual parts and wires - the so called breadboard method. The breadboard prototype is a one-off proof that the design works, it gets commercialised later.
Some designs can be made using commercially available modules, and some wiring and programming - the so called commercial-off-the-shelf (COTS) method. A general rule of thumb is that this will cost less to design, but more to produce.
However, COTS is not versatile and the commercial rules change a lot, as basically the product designer is applying the suppliers electronics to the clients application - the supplier may then later change the price or availability of the equipment.
One increasingly popular method is to use a Programmable Logic Controller (PLC) to do the job. The cost of these has fallen to the point where it makes sense to use a PLC if the expected production volume is small and the nature of a PLC fits the environment where the equipment is to be used - a prime example being a factory already automated with that sort of equipment. It may be a little less flexible - but it is exactly what they use already. The client will know who to contact when there is a problem. With PLC equipment there is probably less danger that the manufacturer will suddenly increase prices, or make the units less available by introducing incompatible new models. Most of the PLC manufacturers are quite large companies who seem to have a committment to their customers through continued maintenance of their product.
A different method again is employed by the larger developers in the world: They have big budgets, split the work into teams, top notch development equipment and established relationships with suppliers so that they can get components made to their specification. They will do extensive simulation, several iterations of prototype, and find simplification of product design into the steps on The design method webpage entirely too quaint.
Whether you think you can or you think you can't, you are usually right. -- Henry Ford (1863-1947)