They mean the same thing, but the usage is different. 兄 is the older term and is somewhat old-fashioned. In modern mandarin it is typically not used by itself, but it still appears in combination with other characters in words like 兄弟姐妹 (siblings) 兄弟 （brother/brothers) 兄妹 (an older brother and younger sister) 老兄 (a familiar form of address between men) Whereas 哥/哥哥 is often used by itself to directly address or refer to an older brother. It is also a component in other words like 表哥 and 哥们儿，but unlike 兄，哥/哥哥 is commonly used by itself.

You can use 哥哥 as elder brother but you can’t use 兄 individually, although 兄means elder brother. 兄弟 means brothers in general . Good friends can call each other 兄弟 你真是我的好兄弟。 you are my good brother (friend) 我们是兄弟。 we are brothers.

兄弟 has two different meanings nowadays. 1. Elderly brother and younger brother. (亲兄弟means they are born from a same parent) 2.buddy / best friends as for guys.

兄means elder brother弟means younger brother. 兄弟means elder brother and younger brother. 兄弟also has another meaning. Typically , It refers to an intimate friendship between men. For example: 我们是好兄弟！ 希望对您有帮助！

兄 is the old way of saying older brother. In modern mandarin, it won't be used by itself, but it still be combined with other characters like 兄弟姐妹, which means siblings. 兄=Older Brother 弟= Younger Brother 姐= Older Sister 妹= Younger Sister