Whitman wanted to express that America is a country constituted by several parts that make a whole that works well. Each individual of the country doing their part is what forms the nation.
A choir is a group of people in which each one has a part to sing that might not be special on their own but form a harmony when they are sung together,
It is a metaphor for the ineffectiveness of colonialism.
A dynamic character is one who goes through some type of important change within themselves. Like a really good character who turns evil, or a really bad character who changes for the better. There is normally a dramatic change in their personality.
