There are many places things get done for refurbishment of seats and interiors - lots in China.

All places doing heavy C and D checks are FAA certified, for US registered airlines regardless of where they do the work.

airwaysmag.com/abcds-aircraft-maintenance/

Delta Techops does lots of work on their own planes and others.

Small airlines won’t be able to afford to run their own heavy check facilities and will certainly outsource.