I write them! It starts with research, research, then a bit of research. I search the web for the person I am interested in and get all the information I can about them. If I can't find any or enough information about a person from the web then it's off to the library and nose into book time. I then read all I can about the person and write the short history of them from what I have read.
I did, on one occasion, actually buy the autobiography (it was not available at the library) of one person that I was researching because I could only find information about them after they had become famous for something else and all the information was just about that one aspect of their life. I discovered some very interesting facts about that individual, along with, not only the invention that aided radio reception from that day on, but also another invention that did a similar thing for film and was a break through for the movie industry!