Help getting a mob to spawn underground

Read a lot of posts with the same issue as me but they're all outdated and I can't find a good explanation on how to do this.

I'm trying to create a mob that spawns only in caves by either being in cave_air or below a certain Y level, but would persist outside of them too (via spawned with command, spawn egg or by leaving the cave). I've attempted to do so with the Custom Spawning Condition but I can't figure out how to get what I'm after. Anyone got tips or even a tutorial that'd help out?