Whilst the seats, toilets, galleys etc are chosen by the airline, they are all from a selection of Boeing approved designs for their aircraft. The seat manufacturers, toilet makers etc all work closely with the manufacturer in finalizing their designs, so Boeing cannot be off the hook completely.
For me the B777 seats on EK and their neighbours down the Shk Zayed Road are much better than those on the A330's I have flown on with the same airlines. This suggests it isn't just airline choice that makes the difference, what is on offer to be chosen is also a factor.