[NativeSon]

Are the planes custom outfitted for the airlines or did AA select available models with smaller facilities?

[Vigilanteman]

All the major airplane factories (Boeing, Airbus, MD, etc.) provide standard design for major components like wings, bodies and engines but allow a wide range of customization for interiors.

This is why you can board exactly the same airline model on American, United, etc. and it will feel like a cattle car or you can fly an airline like Singapore, Cathy Pacific, JAL, etc. and it will feel like you took a time machine back to 1980s style luxury.