How has citizen demand caused the expansion of the federal government?
First, please note that I am answering only part of the question you originally asked because we are not allowed to answer multiple questions in one question. There have been other questions answered that have addressed other parts of your original question. You might try searching for such terms as "supremacy clause" or "federal government" to find some of these.
Although many people tend to say that they hate the federal government and that it is way too big, much of the growth has been driven by demand from the people. As our society has grown and become more complex, there is more demand for government to protect us from the problems that can arise. We have also come to feel that it is the government's duty to protect us from bad luck. Let us look at one example of each of these.
One of the first major government regulatory agencies was the Food and Drug Administration. People demanded such an agency in the early 1900s to protect them from unsafe foods. This had not been a big problem earlier because people were raising their own food or getting it from people they knew. As society became more complex and food came from factories, problems arose and people wanted to be protected.
A second agency that shows demand for government is the Federal Emergency Management Agency (FEMA). FEMA is in the news right not because many people affected by hurricanes on the East Coast want the government to help them repair their homes and communities. The Congress is holding up money for that because they are arguing over budget cutting. People are very upset because they expect the government to fix the problems caused by "acts of God" like the hurricanes.
In these ways, people tend to demand a larger federal government and the political leaders tend to give it to them to fill actual needs or so that they can get the people's votes.