Why is it used "I live on somewhere"? How is different "I live in" and "I live on"?
"I live on Orkney mainland" is used here because Orkney is an island. Since islands are often very small, it is common to use the preposition "on" when talking about someone's residence or current location on a small island. Ex: I live on the Big Island of Hawaii. Ex: She is on an Island in the South Pacific visiting her family. However if you are referring to a cluster of islands, the preposition is usually "in". Ex: I live in Hawaii. Ex: He is in Fiji.