The US have reps who represent the amount of votes from the ppl. For example 1 rep=100 of the ppls votes. Then they vote.
In Athens they don't have reps the votes are counted directly.
To your first question--James Madison is widely known as the architect of the US Constitution (Although in reality it was the result of team work by 56 delegates).
To your second question--The six key principles outlined in the US Constitution are:
1. Popular Sovereignty (i.e. democracy or rule by the people)
2. Limited Government (i.e. again, democracy or the government can only do what the people instructs it to do)
3. Separation of Powers (there are three branches of power: legislative, executive, and judicial AND they are separate from each other)
Which brings me to...
4. Checks and Balances (each branch "checks and balances" the other two branches' power. For ex: the US President can veto congressional bills, Congress can ratifies judicial nominees, and the Supreme Court decides on the constitutionality of congressional bills).
5. Judicial Reviews (the judicial branch reviews governmental actions)
6. Federalism (there is a federal and a state government. These are separate from each other)
