Inquiry regarding regulations on the free export of spare parts to subcontractors of a US-based subsidiary

Company A in Korea sells unassembled finished product 1 to Company B in Korea, and Company B then exports it to its US-based subsidiary, B'. To facilitate the assembly of finished product 1, a service agreement for assembly and REWORK/SUB has been concluded between Company A in Korea and the US subsidiary B'. Company A is freely exporting replacement parts for components damaged during assembly to Company C, which is a subcontractor for B' and is responsible for the actual assembly. Since sending parts after damage occurs would be too late, they essentially send a certain quantity of spare parts periodically. There is no relationship between A and C, and the service agreement between A and B' does not include provisions regarding the free export of replacement parts. I have the following questions: 1) I would like to know if Company A's free export in this manner poses any issues under relevant regulations, and if so, how this should be rectified. 2) I am wondering if it poses a problem for the US-based Company C to import these goods free of charge if it is not specified in the contract between B' and C, and if so, what the lawful import customs clearance procedures would be.