I don't even know if there's a specific word for people that do this kind of task.
Writer. Author. Novelist.
Worldbuilding is the thing a fiction writer does. If you write a contemporary work or a historical work, you "merely" need to develop characters and research specific topics you write about.
There are writers tools for this that aim for fiction writers. Maybe those can be of help to you.
My advise would be to write the story/lore first and worry about fancy words later. You do not need fancy big words for the sake of it. The need for them would arise naturally. To pick up the example above, those "skyborn" and "sky islands". Such do not exist. But there obviously was a plot device to have islands that somehow are not in the water, but in the sky. Calling them sky islands is not a fancy word. It just sounds fancy, if you are not familiar with English. Same for "groundbound" or "skyborn". Both likely were needed by the plot device of dividing inhabitants of the sky and inhabitants of the (ground) earth by using different words for them.
I suggest you read the first page of this comic. It helps if you are familar with pen and paper role playing games. Or ttrpgs as they are called on Itch. https://darthsanddroids.net/episodes/0001.html