What determines egg moves to be egg moves?

Question pertains to Gen III only please. If it extends into IV+ just mention.

Been having trouble finding a more proper explanation as to how egg moves work.. and by work, I mean how they’re established in the first place.

Are they merely a separate move list that Pokemon have similar to their level-up counterpart where it focuses on the breeding mechanics in order to be obtained?Or are egg moves some loosely defined moveset that really could change if a Pokemon were to simply have a move it shouldn’t in its move pool?

My hunch/assumption is that egg moves are merely some sort of separate hard-coded move list similar to how the level-up moves are arranged for Pokemon. I’m trying to refrain from requesting a ROM’s point-of-view, but if that’s the case I am all ears. The breeding sections explained in most sites do not explain the little detail I’m trying to clear up in my mind. I guess I am secretly hoping I won’t be confined to the usual TM/tutor/egg list you see online.

A secondary/probably the same question to bring up is what DEFINES an egg move? Is it an obtainable non-TM AND non-level up move? If so, then why does Hoppip have reflect as an Egg move and not as a TM-learnable move? Would that then make the definition of an egg move to be ANY move that is available to the Pokemon that isn’t found in its level/TM list? As Bulbapedia says:

Egg Moves often (but not always) cannot be learned through other ways.

Even if that were the case-what determines the egg moves to be there in the first place?

Is it hard-coded or not?

Pleaselog inorregisterto add a comment.

the way it works is if one of the parents have the passable move, like inceneroar being able to pass on fake out to litten, the move is an egg move. its hard coded, so if the parent has a passable move, it will be passed on. this applys to every generation that egg moves are a thing. hope I helped.