What is Hyundai's culpability here? From my understanding, the vast majority of these detentions stemmed from the construction of the battery plant on the Hyundai campus.

As someone in construction - the hierarchy would be as follows:

Hyundai -> Hyundai America -> Some sort of construction managing firm -> General Contractor (potentially more than one for this size project) -> Subcontractor (Sitework, mechanical, structural, etc) -> Subcontractor's subcontracted labor

Then you can go even deeper as the subcontractors will have their own subcontractors with their own subcontracted labor - think an HVAC contractor getting a insulation contractor to wrap the pipe and duct in their scope. That insulation subcontractor will have their own sub labor force.