In Japan, by definition, an “otaku” is essentially someone who is obsessed with a specific subject and dedicates an overwhelming amount of time to it. For instance, a “train otaku” is someone who spends all their free time working on model trains, to the detriment of their responsibilities.
However, on the Japanese-speaking Internet (mainly 2ch), and to a lesser extent in English-speaking circles (4chan’s /a/), “Otaku” was co-opted from an insult to the tongue-in-cheek name of the subcultures surrounding anime, manga, video games, idols, and nowadays Vtubers. The dedicated “Otaku subculture” is essentially all forms Japanese media, but especially manga.
It’s best to think of it like the real world definition of autism compared to 4chan’s definition of autism.
Huh? I thought people that are obsessed with trains can also be referred to as otakus so it is not exclusive to Japanese media.