Each city=3 base unhappiness (less on huge maps)
Each population adds 1 unhappiness
Happiness from buildings (non-Wonder) is limited to the population of the city they are in
So a 1 pop city with a Colluseum and a Zoo will add
3 unhappiness from city
1 unhappiness from population
1 happiness from buildings (2+2=4 happiness from buildings is available... but the city only has a pop of 1)
So happiness/unhappiness is for your Whole empire (like Faith, Gold, Culture, Tourism)
Production and Food only go to the city they are in.
Buildings only affect the city they are in (since you can have 1 in each city)... if you have a colluseum in your capital, that is 2 Happiness... regardless of your # of cities.. to get more happiness from colluseums, build colluseums in ALL of your cities. (same with Markets, Shrines, Monuments, etc.)