This is gonna be a long one! I'm going to cover the main story, since the story's Epilogue arc is a bit of a work in progress!
Ren is a high school student for most of the main story. I could see him getting some side jobs, but they'll have to be part-time since he's a full time student.
Haewon's also a full time student (either in high school or undergrad). She's built different and works a plethora of jobs to make money. I could see her doing most of her jobs on-campus since she's there all the time, maybe at the school gym/pool or as a tutor or TA. Haewon also picks up gigs to perform and keep up with her hobby outside of her studies.
Ru's work is...classified. Beyond their activities as a full time student (like Haewon, so either high school or undergrad depending on the arc), Ru doesn't tell anyone about their job, not even Haru.
Haru works graveyard shifts (aha, graveyard shifts for a ghost companion) at local grocery stores, cafes, or convenience stores. Companions/magical beings don't need to sleep like humans do, so Haru takes advantage of that to get that nightshift bonus pay.
Wren doesn't work, unless you count stealing or being a general menace to society as a profession.
Gordet also doesn't work, unless you count being trapped due to his hubris and maintaining the psychological stability and memory of his Muse as a profession.