Lieve Dijkkijk -
I honestly doubt that airplane manufacturers design airplane for strictly one airline, they all hope to sell many, to many other airlines, but, it may end that it very much looks that way...
The Brittania was mostly acquired by BOAC, but there were a few other operators who had them, in Africa and North America. Soviet airplanes were mostly acquired for use by Aeroflot, but many countries of the then Soviet "block" acquired them as well... Eastearn Europe, North Korea, China, SE
Asia, some even in Africa, then Cuba... I see a few old Antonovs in Peru.
I think of the DC4 conversion to ATL98, for Channel Air Bridge to carry cars.
Pretty much like the Bristol 170 was doing.
The Canadair CC109 was designed for the RCAF, but eventually sold many as swing-tail CL44 versions, to cargo carriers... Loftleidir Icelandic was the only airline operating the passenger versions of the CL44...
One more recent example is the Dassault Mercure, used only by Air Inter...
But I am sure that Dassault tried to sell it to other airlines.