However, neither of them exist in this world.
Mt. Fuji is registered as a World Heritage Site.
He lives in his own world and doesn’t want to leave it.
The generations (and society!) that make up the world create our world. So this is the World, society, or the Universe.
This word is used more like "the biggest dog in the world" or "the fattest cat in the world." For example, you'll hear words like 世界一番 (number one in the world) thrown around a lot.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.