The interiors are ordered and paid for separately by the owner of the aircraft. Often they are 'line-fit' by BA/AB as the aircraft is built, sometimes they are retrofit by an appointed agency after delivery. BA/AB don't like to have to do complex linefits of anything, so they only work with a standard set of seat suppliers and only approve of certain types for each aircraft type. The same goes for IFE, galleys and other things that go into aircraft. Linefits that are too complex hold up the speed at which aircraft move down the line, require additional training of workers etc. For example LH Technik installs the P/E seat from ZIM in MUC when a new A359 is delivered.

If you look closely at the pictures of C cabins, AI/LO/ET/TK and others went for the same Zodiac seat that is a linefit option. Air China uses the standard version of the B/E (now RC) Diamond seat that LH has customised. Hainan had a standard version of the RC Super Diamond installed in the B789. If LH picks the standard version from the suppliers no one will object.