How it fits together
The Constitution creates the national government and divides its work among Congress, the president, and the courts. It also limits what government may do. This is why a republic, representative democracy, and constitutional government are connected ideas rather than competing answers.
A closer look
The original Constitution was written in 1787. Its first ten amendments, ratified in 1791, are the Bill of Rights. An amendment changes the constitutional framework; an ordinary law operates within that framework. Article V makes constitutional amendments harder to adopt than ordinary laws.