Unknown Nationality?

If you are born on international waters, or on a plane flying over international waters, what country are you a citizen of? I mean, who decides? Is it the country you are nearest to, or is it which country owns the boat/plane you were on, or where your parents are from, or are you given a UN citizenship? How does this work?

depends on the rules of various countries.
if you are born of an american parent, you are an american, no matter where you are born.
and if you are born in the u.s., you are a u.s. citizen, no matter where your parents are citizens.

United Nations citizenship? Where would you live in that case? Would it mean you had a passport to the whole world?
Where would your birth certificate be kept?

United Nations Citizenship! My passprot would be available in the world. And that is where my birth certificate would be available also…