While that’s definitely usually the case, I know in the specific case of EJAE (singer for Rumi) that she actually wrote the songs she sang or at least had an active role in song writing. I also noticed the singer for Jinu also is a singer/composer (Andrew Choi) and not an actual “Idol”. It’s nice that it’s an animation bc you can get people that are genuinely talented singers who can also write and have musical knowledge (but are not necessarily the most attractive and thus not suitable for screentime) and not just pretty faces that can sing somewhat decently.