It's pretty simple. If a person posts more than two books officially, it becomes a series and they own the title and the ideas in it. However, every single industry goes by different copy write laws. For instance, in research, you can't use three words of someone else or that is considered stealing their work.
In fiction, it is much more nuanced, and don't even get started about intellectual property. Basically, you can not only own your work, but the idea of your work also belongs to you. For instances, the name of characters can be used in copywrite cases, and the list goes on and on.
However, for the purposes of webnovel it's much simpler and all comes down to claims. If you file a DMCA claim, and are the author of the original work, then they will have to take it down. Otherwise, if you are not the person who wrote the book, and the person of the book never finds out you did it, they can't technically take it down.
Also, this site probably has to follow international guidelines for copywrite law, so.... yeah,, it's complicated but all comes down to the original author's interpretation of the said authors work.
Hope that helps