One of the richest men in the world is the appositive.
It has a comma that separates it from the rest of the sentence and the sentence makes sense without it as "Bill Gates started Microsoft."
If Bill Gates had a comma after it, it would be the apositive and the sentence would be "One of the richest men in the world started Microsoft," but since there is only one comma the appositve is "One of the richest men in the world."
Harry Potter is famous around the world because it is magical, when you read those books it's like entering the wizarding world and experiencing the things Harry does. Kids like Harry Potter because like i said it's magical and brings out the wizard in all of us, adults like it for probably the same reason, when you read or watch Harry Potter you are entering a world you never thought existed. Hope this helps :)
