I don't know about others, but my seatmaps (and I've made quite a few, even more than I've posted) are all 100% original. I have never copied anything from anywhere.
I use multiple references (from real-world seatmap images to engineering diagrams to ACAPS documents) to get the shape and size of the fuselage right. After doing that, I basically have templates for each aircraft type that I can put different cabin configurations into depending on what airline I'm making a seatmap for. I then create seats by hand, turn them into smart objects, and fill the cabin with them as needed (organizing into groups by class and row, for convenience). It's all hand-made in photoshop - there are no ready-to-use templates, and as far as I know also no post on the forum telling you how to make them (well until this one, that is).
I'm not sure how other make their seatmaps. I know some take the lazy way out, and basically recolor an existing seatguru image or something. Personally, I have very little respect for that method. Then there are those who actually put in the effort and make fully custom seatmaps like I do - obviously I don't know exactly how they do it, but I assume it's something similar to what I described above. Looking through the gallery, you will probably notice that different creators have very different styles. Personally I love that, and I hope you find your own unique 'look' if you get into this seriously. Basically, making a good seatmap requires a effort and lot of attention to detail, but if you're ready to put that in, the results can be very very nice. Good luck!