If there's an age difference of a year or more, yo refer to them as 사촌 형/오빠/언니/누나 or 사촌 동생. But when you call them, you say 형, 오빠, etc. without the 사촌 part if they are older, or call them by name if they're younger. If they are the same age as you, you refer to them as just 사촌 and address them by name when they're young. As you get older, you use the names less and less. We tend to use indirect words like so-and-so's father or so-and-so's mother once they have kids of their own. We generally don't use names of older people in the family. We don't use a term like 사촌 친구, as 사촌 already means something different than a simple friend. If you say 사촌 친구, people might think 사촌의 친구, a friend of your cousin. But in terms of how you associate with them, a young, similar aged 사촌 is like a friend in most aspects.