For example "The Sino-Japanese wars and "The Sino-Soviet border conflict" the word Sino is used to represent china, why is it not "The Chinese-Japanese wars" and how did it become this way?
It's from the Latin "Sinae". There are other countries and conflicts that follow the same model, particularly when using two countries in a hyphenated fashion, e.g. The Franco-Prussian War, the Anglo-Afghan war, the Russo-Turkish war, the Serbo-Bulgarian war, the Italo-Abyssinian war, the Dano-Swedish war, and so on.