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 several carriers who only perform maintenence in Mexico and South America to save money and avoid unexpected FAA peeks at the maintenance records.
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.