How do they upholster this kind of seat?
The quick version!
The bottom bun is usually done as three separate pieces. Bolsters, and center pad. They are almost always held down between each other with a long wire and hog rings.
The seat back is a little different. The two side bolsters and the back cover are sewn together to form one piece. this is then wrapped around the back of the seat and held in a similar fashion as the bottom is. The center pad with the headrest area, are again a separate piece but done in a similar fashion as the bottom.
I have done several Ricaro and Flow-Fit seats that are built this way.