How is there interdependence in The Giver by Lois Lowry?

The concept of interdependence means relying on someone else.  In Jonas’s community, it is related to the concept of Sameness.  Everyone is the same, everything is controlled, and people need to learn to rely on the community.  This means that as children they need to learn to rely on other children. 

The community teaches interdependence by making children rely on each other.  From the age of Four to Six, the children’s jackets have buttons in the back so that they will have to help each other dress.  When they are Sevens, they are considered old enough to button their own jackets. 

The little girl nodded and looked down at herself, at the jacket with its row of large buttons that designated her as a Seven. Fours, Fives, and Sixes all wore jackets that fastened down the back so that they would have to help each other dress and would learn interdependence. (Ch. 6) 

In addition to clothing, the people of the community rely on each other in different ways.  Mostly, everyone relies on the community.  Spouses, children, and jobs are all provided by the community.  Individual choice is not really a concept.  Everyone dresses alike, and due to genetic manipulation they even look alike.  With rare exceptions they all have the same skin, eye, and hair color. 

Everyone in the community also depends on the Receiver of Memory.  This is the person who holds the community’s collective memories and history, of which all other community members remain ignorant.  There are not even books in the community other than instruction manuals. 

The Giver shook his head. "Jonas," he said, "the community has depended, all these generations, back and back and back, on a resident Receiver to hold their memories for them. I've turned over many of them to you in the past year. And I can't take them back. There's no way for me to get them back if I have given them. (Ch. 20) 

Since everything in the community is collective, and the community takes care of everyone’s needs, it never occurs to anyone to question anything.  Things just are the way they are, and if you are a citizen of the community you follow the strict rules.  To not do so is socially unacceptable and can result in grave consequences, such as release.