How to make Japanese seaweed rice balls? What materials are prepared? Why add rice vinegar?

(Korean) It doesn't matter. It doesn't matter whether rice vinegar is added or not. It just improved the taste and made it more fragrant. First, add some things you like to eat to the cooked rice, such as dried walnuts or some vegetables and diced ham. After adding it, spread it on a large piece of seaweed (seaweed) and roll it up slowly. Don't break the crisp seaweed slices (be careful). After it is rolled, it is quickly cut into small pieces with a knife, and it is finished. (Japanese style) Add the same thing to the front and knead the rice into balls at the back. Before that, tear the seaweed into pieces and put it in a bowl. Then put the rice ball in a bowl with seaweed and let it stick with seaweed slices. These are the simplest and most labor-saving methods, and you can also buy them directly.