Was I the only one who learnt about this before the other weird meaning even thought I don’t really know Japanese
Maybe because it sounds like the meme 'onii-chan' in cute voice. And anyway, -chan doesn't actually specialize for kids-use. -chan is similar to -kun (for male), used by someone who's close or considered or want to be close to the other party [at least, this is what I get from skimming through honorifics)
no, this is what i learnt in formal international language classes, like, these are in my textbook.
no offense, just mentioning bc I'm stating what I've learned for the past years of taking japanese language classes
like, yeah, the informal of the daily usage of it in japan itself might be different, but the formal understanding of it is what's in the textbooks
onii-chan literally meant "brother" / like the equivalent of "hyung/oppa" in korean . while yes, in korean, hyung was commonly used from male to male, and oppa is used from female to male.
there's nothing wrong with it although the more formal use of it will be onii-san. bc the particle "chan'' is usually used for kids.