For Arcane Archer, when a creature is hit by piercing arrow but passes their save do they take arrow damage (1d8,1d10,etc) + half of 1d6 + modifiers or does it take half of arrow damage + 1d6 + modifiers?
Half of (arrow + 1d6 + modifier).
The text of piercing arrow says:
On a failed save, a creature takes damage as if it were hit by the arrow, plus an extra 1d6 piercing damage. On a successful save, a target takes half as much damage.
Compare this to fireball:
A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.
When a creature succeeds a save and takes half damage, it means that they take half of the damage they would have taken on a failure.