Engines are weirder. My understanding is that the airline specs out what the engine requirements are, and any engine manufacturer is allowed to try to meet that

Engines are weirder. My understanding is that the airline specs out what the engine requirements are, and any engine manufacturer is allowed to try to meet that requirement. Customers get to decide which engine to fit and negotiate for them separately from the aircraft. Sometimes there's only one choice, but other times gives you a chance to have the same (or similar) engine to maintain across your fleet (or avoid that to avoid a total grounding) and whatever other reasons to choose one over the other. At least that's what the A380 tour in Toulouse said. Kinda makes sense to at least keep the big aircraft builders from throwing too much of their weight around.