i heard kubera 'webtoon' was adapted from kubera light novel something (whatever that volume 3 stands for) but there is not a word like about the publisher here? and does anyone know any details about that?

Pretty sure Kubera started out as a webtoon. The volumes are probably just a printed collection.

"People don't change, they just give in."Kubera is the best.同人音楽 (doujin music)

Actually Currygom was planning to write Kubera as a novel, but it ended up at a webcomic instead. I don't know the reason's why, but I am happy that it became a webcomic.

Currygom did posted a side-novel on her blog called The Finite.

The finite is about Kasak's parents love story right?

Webtoons, Manga, Manhwa!

Yes, "The finite" is a lot about Kasak's parent's love story but it also gives much more details on the main story. A LOT OF DETAILS.
She posted the novel during season 1 and then abandoned it to begin it again during season 2 with some changes.This second attempt was stalled for later as it was taking too much of her time to post the webtoon and the novel chapters together. She had said that she would pick it up later.
If you wish to read it in English, you could find some posts by "Mizura" on the Batoto forum as a summary of this novel.