Hello, fellow Hibike! Euphonium fans ! (and the others too, of course)

First, I have to say that I've just finished all episodes of Hibike! Euphonium 2 days ago, so maybe it's just a temporary fever, but at least for now, this anime is my favourite! I love all the aspects of the anime, from the characters to the music. So, because I wanted to know all about Hibike! Euphonium, I naturally came on this Wikia. And I'm surprised by the amount of informations that are available! So a question passed through my mind: where are these kind of details come from? I don't understand how the precise height, the blood type, and other informations like that can be known. Even in the light novels, I would be surprised that kind of thing appears inside.

Now, I go back listening the Crescent Moon Dance, for around the 54th time :D