Well, without having listened to the Rumble Fish OST in New Gisaeng Story specifically, we can assume that it was composed to complement the narrative. It might have a variety of musical styles within it. For example, there could be slow, ballad - like pieces for the romantic or sad parts of the story, and then more energetic tracks for the action or tense scenes. Also, the voices in the OST could be carefully selected to match the characters' personalities, whether it's a soft - voiced singer for a gentle character or a more powerful voice for a strong - willed one.
The answer is yes. Made in Abyss began as a novel. This original form allowed the creator to lay out the core ideas and storylines that were later brought to life in other visual and narrative formats.
The Made in Abyss manga is on hiatus for now. Maybe the author needs time to flesh out the story or there could be other behind-the-scenes factors affecting the schedule.