Answer:
Gerrymandering is an old, vicious practice of democratic politics. Simply defined, it is setting the limits of voting districts to favor a political party, it´s usually done by those who control the legislative branch or a body charged with defining such limits in order to help their party win votes and districts. Perhaps we can all it politically-biased mapping.
How is it done and what the census has to do with it?
Setting the administrative division of a country, a province or a city is usually the business of an executive branch or the legislative branch, it depends on each country´s system. A census provides detailed information on the people of a country, province, state, county, district, etc. In each administrative unit, people are grouped in categories such as gender, age, profession, income, etc. Each political party knows where its supporters live and who they are. So, they use the census to collect information and to attempt to map districts in the most advantageous way to themselves, so that large numbers of their voters will be registered in districts they are interested in; this will assure their victory in elections. It is usually done ahead of elections. As I said, this is an old trick by all political parties in democratic politics.
I have answered your third question.
Explanation:
