Resolved questions
Explain the use of 其 in 钓鱼岛及其附属岛屿

钓鱼岛 及其 附属岛屿 = 钓鱼岛 和它的 附属岛屿
in official speech or article and other formal conversation etc, we say 及其 just like this example,
but in informal conversation, we say 和他/她/它的 instead of 及其.

anyway, we can just hear 及其 in news, newspaper and formal articles.
we NEVER say it in informal conversation.
for example, u want to introduce ur friend Tom and his parents to your mom, if u say: 这是Tom及其父母. this is totally weird.....u must say 这是Tom和他的父母。