I would also say engine too.
A modern turbofan can run for 20,000+ hours on-wing before removal for overhaul. That's longer than many car engines last in total.
The operators, such as Delta, do not actually own engines on the aircraft they fly, even though they own the aircraft. The engines are rented from e.g. Pratt & Whitney along with a maintenance contract. That said, that engines are in fact installed at the factory.
Which is not to say that commercial jets can take any old engine. Even something like this 767 that was split between GE and P&W have specific structures related to the original engine.