All of the above. Parts may be real / legit parts that exceeded their official lifespan, or "3rd shift" parts made at the same factory with mostly or entirely

All of the above. Parts may be real / legit parts that exceeded their official lifespan, or "3rd shift" parts made at the same factory with mostly or entirely the same materials, just not run through QA to save money and given fake testing certifications. Could also just be a knockoff that's good enough most of the time. Large flag carriers like Delta also have real staff and processes, and it looks like they're doing inspections and sometimes catching bogus parts; smaller organizations may not do that.