Ok, so first I'm going to clarify my opinion because its not what really wanted to say
Yes I truly respect the authors. They worked so hard to make the stuff that we love. And its like an insult to them when illegal manga sites upload there creation
But. As a broke person myself mangago is like a gift from heaven. And of course I'll go support t...... reply
Besides the question, I also want to learn more about it, from what I can grasp, they are fan made, and are about the creators own personal or popular shipping between two characters from their favorite Anime/manga(all types)
But, this is just my own speculation. It might not be true
I've never read one and know the plot behind it, mostly because I don't want a lot of Anime. But I do think there interesting I guess?