Escalating scandal grips airlines including American and Southwest, as nearly 100 planes find fake parts from company with fake employees that vanished overnight::Why are so many flights getting canceled or delayed? Blame a mysterious British supplier accused of falsified documents for plane components.
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.
https://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.