What is ok and not ok to do in Japan

I see everywhere some facts about japan, like for example you can't eat while walking, can't talk in the shinkansen, ect. But what are the true things that you can't do in Japan ? I don't want to accidentaly do something rude 😅 And what are things that I need to know ? I saw that not all toilets there had paper and not all showers had soap. True or not ? I see a lot the "you can't show your shoulders" too, is that true ?