Most of them are from books/light novels or real mythology. If you read them, you’ll see that those known characters are truly unbeatable,indestructible and all-powerful. Also I think all of them are mentioned as omnipotent in their respective universe

Because the authors usually say so.

I'm asking this because I saw people on YouTube saying that the lord of nightmares is not omnipotent do how do they know that LON is omnipotent

They are no true users of omnipotence. But the characters in the wiki are stated to be omnipotent in their fictions.

So there are no omnipotent characters in fiction?