Developer here.
The DCS Modules (ie, Aircraft and what ever else) work using a plugin system.
What this means is the DCS World engine allows a developer to access certain functions and code within the engine itself. This is known as an API, Application Programming Interface. The functions that the API exposes to the developer are whats used to interact with the engine.
So for example there may be a function that accepts inputs regarding flight modelling and what the aircraft is doing. These inputs are passed along to the engine which produces the result in game.
It's fairly straight forward to 'convert' one module to work with a new engine. All the engine has to do is provide exactly the same interface. As far as the module is concerned nothing has changed. It just 'plugs in' to the engine and works.
Much like connecting a device to your PC via USB, your plug in your joystick and the PC accepts those inputs. If you connect a new joystick you don't have to change your PC, or conversely if your change your PC you don't have to buy a new joystick.
So as long as the API provides the expected functions the module needs, everything is interpreted by the engine.