transition between hardwood plank and molding on stairs

Maybe a stained or painted quarter round or some sort of small fancy, decorative moulding.

Depending on environmental conditions, the wood may expand and contract with the moisture in the air, so the gap is useful.

Decorative moulding is one option, though you may have difficulty making it look good as it curves around the nosing.

An alternative is to find some wood coloured decorators caulk and apply a small bead in the gap. You should be able to match it to the hardwood used on the stairs. This will be flexible and provide a consistent finish down the flight of stairs (as long as you apply it neatly...).

That's a pre finished engineered, very stable. He should have pre glues planks together to make full size treads and risers. Then each step can be templeted. Then each tread and riser get glued in and blind nail. I full spread glue mine on but a nice serpentine of construction adhesive works too. They look awful, be sure to get him back out.