Okay, this question is a little tricky.
The simple answer is that there is a "tag" in the NEF file that names the camera. Let's go back in time for a moment for a moment to see a non-sensical difference in NEF files: the D70 and D70s. Other than the tag saying D70 in one type of NEF and D70s in the other, these files are identical in layout and information. A programmer could have used the line: IF CAMERA_TAG = "D70" OR "D70S" THEN… to identify code to decipher those files.
But let's get to the tricky part. There are a number of things that may change from camera to camera: pixels (overall number and layout), Bayer filtration, AA filtration (or lack thereof), compression, bit depth, white balance (neutral point), and more. Each of these have implications for the conversion routines.
Adobe's answer was DNG, an attempt to further standardize a standard (all NEF files are of TIFF-EP type, which is a standard). Additional tags were defined in DNG to deal with many of these changes that could occur, and a method was provided for adding new things should they be necessary. Nikon has no real interest in DNG, as to provide full DNG data would reveal additional details about how their sensors render information, making Capture NX-D potentially no longer unique. It doesn't help that there's a fairly long-standing feud between Adobe and the Japanese camera companies, which apparently was started by Adobe's aggressive stance and costs to the camera companies during the early digital days (remember, Photoshop was one of the first programs to be able to work directly with scanners and digital devices).
DNG doesn't fully solve all problems, either, though if everyone standardized on it we could move on to other problems.
The bottom line is that as long as digital cameras keep getting new sensors, filtration, and electronics, there will be information that changes that a raw conversion program needs to know about. The current Nikon policy (as well as Canon's and Sony's, too), is to iterate raw files with each new camera.
Unfortunately, what this means is that every time a new camera comes out, there is a delay in supporting it by software vendors, as they scramble to deal with whatever changes there might have been. The tricky part of this is that some software vendors disappear over time, and thus so does support for newer cameras: you have to move to another software product. The most distressing of these is Adobe's Photoshop policy. Camera Raw is updated with every Photoshop CC and Lightroom Classic CC (and Photoshop Elements) version. Thus, if you want to keep up with a new camera, you may find that your perfectly fine older version of Photoshop needs to be replaced by a subscription to the Adobe Photography Plan.
