I am kinda interested in books about how to do world-building but if you know any goods on how to write novels that'd be great. It's better if it's from Japan/otaku-related.

If I think of a good story I just write it down on my phone and then as the days go by and I think about it more I flesh it out. I dont relly have any story I believe to be worthy of publishing but that's how I do it

Read a lot, think about what you liked in the things you've read, come up with a general idea of what you want to write, write it all down.

world building comunties are very inspiring

i don't have any books to recommend unfortunately but i do have a little bit of advice. i think that first, the world needs to be created, populated, given strict rules and filled with problems that may or may not be tied to the story that you want to tell within it. then the story can be told, but it should never break or bend the rules of the world, and it should often be derailed by coming into contact with some of the problems of the world. the rules of your world can get as crazy as you'd like, what matters is following them. i also believe that it is important to represent man's struggle against nature (or his dominion over it) if the world contains humans or human analogues since this struggle is what defines the way of living in any particular region. survival is what shapes culture. an important part of this representation of man's struggle against nature is the definition of his position within the food chain. maybe humans are few and weak, living in tents and never staying in one place for long, for fear of what stalks them. maybe humans are many and powerful, building cities, enslaving lower lifeforms and dreaming up elaborate myths. the level of control that is exercised over nature can complicate or simplify some obstacles that may be faced. if you're doing fantasy, it's also important to define the rules of magic, sorcery, thaumaturgy, whatever you wanna call it. maybe they have some degree of control over the forces of magic. maybe they're at the mercy of those forces. maybe only a select few can use magic, either because of predisposition or because of a systematization of magic and the restricted access to learning material, or maybe it's because humans have no innate magical potential and their only access to magic is through rare, naturally occurring or man-made magical items. finally, and most important of all, you need to have fun creating your world. sometimes it's better to establish nonsensical or silly rules if you think that you'll be able to use them in fun ways later on.

p.s.: if you're scratching your head trying to find inspiration for the culture of a specific people, you might not need to look too far. take a look at the people around you, see what can be adapted, transformed or inverted to fit in your world. look at their speech, their songs, their stories, their laws, their celebrations and their games. look at their failures, their lies, their crimes and other wrongdoings.
p.p.s.: try drawing a "worldbuilding pyramid". start by outlining the lower levels. you're free to choose what you think matters most as a base for the rest. i chose geography. then you can work your way up to the less elementary but not any less important stuff, like culture and technology. by defining the basics early, you can avoid facing dissonance later on when you get around to implementing specific locations or story beats.