What are some questions that are addressed in the study of competitive and non-competitive markets?

I- Why do only a few companies compete in some markets, while many compete in others?
II- Is the welfare of consumers necessarily greater if there are many companies?
III- Should the State intervene in markets where there are only a few companies?
IV- Why do prices rise or fall rapidly in some markets, while in others they hardly vary?
V- What markets provide the best opportunities for entrepreneurial individuals considering starting a business?