You are correct, and a pattern has emerged: there are the airlines that differentiates themselves on service that is tied to their country of origin (Singapore Airlines vs Emirates, both fly A380, but definitely different feel to them), and then there are no-frills airlines.
The collaboration you speak of are mostly to do with cabin sizes, how many seats, which variants to make to satisfy the majority of customers -- these can all be changed when the main parts of the plane stay the same: e.g. fuselage, engine, etc. Rarely do an airline get Airbus/Boeing to change main parts of the planes: one exception is EasyJet when they bought an insane number (200?) of A320s from Airbus: Airbus made a fuselage specially designed for EasyJet that had more seats and they had to move their doors.
If you look at almost any modern airliner's development cycle, you will see that a significant period of time was spent simply developing the specifications in collaboration with the airlines. I am not familiar with any commercial jet that has been developed without commitments from airlines to buy it.
Even the DC-3 was developed in response to a request from TWA for something similar to the Boeing 247.
The collaboration you speak of are mostly to do with cabin sizes, how many seats, which variants to make to satisfy the majority of customers -- these can all be changed when the main parts of the plane stay the same: e.g. fuselage, engine, etc. Rarely do an airline get Airbus/Boeing to change main parts of the planes: one exception is EasyJet when they bought an insane number (200?) of A320s from Airbus: Airbus made a fuselage specially designed for EasyJet that had more seats and they had to move their doors.