My background is in physics so I have a very basic understanding of circuit design. I was just wondering, If someone were to design a printed circuit board (from scratch) how do they know that it will function the way it should? and also how do they know that the printed circuit board will satisfy all the conditions for it's use? For example if I wanted to build a calculator or a digital watch from scratch I would need to use software to make a schematic of the PCB but, how do I figure out how to arrange the components so that the PCB actually does what I want it to and better yet how do I figure out what components to use? Sorry if this was long and confusing.