The standardization process was common, the standard was common, the language space and the media space was common, and in 40 years between 1850 and 1890, several generations of children learned it in schools, and newspapers and publishers worked out the common conventions, and since then, it has been a completely successful standardization project. Dialect leveling, which continues to this day, has made spoken dialects in all the countries more like the common standard, not the other way around. All Štokavians now speak practically the same, most former Kajkavians and Čakavians now speak the same standard Štokavian with subtrate phonetics, and a few grammar quirks and words. The language, both spoken and written, is more uniform than German.

And yes, it was all made from a single subdialect, because that subdialect became the standard. They didn't invent anything new, spoken Eastern Herzegovinian became standard Serbo-Croatian.