reto question for eyes and hair.

My intention.
Character from blender to Unity. Eye movement and hair/eyebrows. Swapable hair styles and eye colors.

What I have done so far.
Full mesh of the body (except for hair and eyes), completed Reto of it, and UV maps.

My hang up.
I don’t know if eyes need to be part of the mesh/reto or need to be separate objects? What about hair? Do I make a particle of the hair, turn it into a mesh and then reto it? or do I just make it on the reto? or do I make it a completely different object?

All the videos and guides just seem to tell me the basics of making a Reto, but I’m past that stage.
Any advice would be appreciated. Thank you.