The type, the airline, and the specific aircraft all have to be certified for ETOPS. The airframe/engine combination has to be certified to demonstrate a certain engine reliability as well as the ability to operate up to whatever flight time with an engine out.

The airline needs to be certified to have specific dispatch procedures that are relevant to operating ETOPS flights.

The airframe has specific maintenance standards related to operating with ETOPS, as well as requiring additional safety equipment on board that would not be required for non-ETOPS flights. American Airlines got in trouble a few years ago because someone messed up and dispatched a non-ETOPS A321 on a flight to Hawaii (they have a mix of ETOPS and non-ETOPS certified planes). Numerous folks completely overlooked the fact that the plane involved wasn’t certified for that flight, even though it was otherwise almost identical to a plane that would have been allowed to make the flight (when they discovered the error in Hawaii, they cancelled the return flight and had to ferry the plane back to the mainland).