Yeah, doujinshis are mostly spinoffs
Doujinshi (同人誌), also romanized as dōjinshi, is the Japanese term for self-published print works, such as magazines, manga, and novels. Part of a wider category of doujin (self-published) works, doujinshi are often derivative of existing works and created by amateurs, though some professional artists participate in order to publish material outside the regular industry.
*NOT NECESSARILY* a spinoff
This might be quite confusing. I'll substitute doujinshi with its proper English equivalent term, so that I may put my point across. "Self-published works are mostly spinoffs" is quite erroneous. It would make more sense to say "Most spinoffs are self-published works". Do you know why most of the spin-offs and fan-made works are self-published? This is because if these kind of works were officially published in a regular industry, they will be hit by copyright or plagiarism laws since the art and story they are based on are intellectual properties.
This is just similar to webtoons and manhwas. Even when a scroll-down format comic does not originate from Korea, they are still erroneously referred to as manhwas since "Webtoons are mostly manhwas".
My point is, it's technically not right to directly assume and associate the word doujinshi to spin-offs and fan-made works, since doujinshis or self-published works are not only limited to such things.
what is this a dj of?