Fan fiction Plus Fan art
Doujinshi is something that is far from the original counterpart. It is therefore a fan art or a fan fiction which was written and drawn by a certain artist, or even a group of artists called a circle. When I explore this kind of topic, another universe has been tackled.
There are several kinds of Doujinshi. Namely, there are fan art series which were compiled into one book. The other tells a fictitious story which was very far somehow (or doesn’t have any relation) from the Anime or the original Manga series. And the other was the same from the last sentence, but in a hentai genre (usually H-Doujinshi).
As mentioned above, there are Doujinshi which are made for young audiences, or even teens. And there are some that are made for adults (18 or above). Usually, adult-only Doujinshi has lots of sexual-oriented content.
My experience to this has begun last year, when a friend of mine invited me into a group in which they discuss some ‘for-fun’ topics and memes, and as well as some lewd or sexually-oriented fanarts. I don’t care if they were bullying a certain Anime character there (like ‘insert name here’ is a worst guy/girl, etc.), so as long as they were not even real. And yep, they were doing it for fun purposes.
The question was why people do Doujinshi from some Manga or Anime series that was previously or recently aired or published? In my opinion and experience, they were just doing it for fun because something they had watched or read did not somehow achieved what they want to happen. The thing is that they tend to do some ‘alternate’ story which will resolve that conflict of that Anime series. Again, that alternate story of them is very far from the original — their drawing art is the clue.
Copyright can be a problem or issue to them because they were just derivative from the original. What artists from the circle do, probably, was that they sell their Doujins to the market, then they got money, and then they pay royalties to the original creators of the Anime and Manga series that they derived.
How about those H-Doujins that I had seen or read? Recently, I had developed this ‘viewing ability’ after seeing a certain H-Doujin that was saved somewhere in my USB flash disk or external HDD. What happened was that I began searching them for more, and they had become my ‘research’ materials which was used for my private session (You know it).
To summarize, the world of Doujinshi was expanding wider and wider. As long as Manga and Anime exists, there are Doujins which were existing from side to side. Both H and non-H are existent, and still they were fictitious from the original.