Chinese spelling is also determined by meaning. 是、事、and 式 are all pronounced shì. Which one you use depends on whether you mean the copula, an event/business/matter, or a test.

I see that your examples are getting at how character components work in Chinese, but it's oversimplistic. 女（nǚ) means "woman", but 娘 (niáng) means "mother". It's composed of the functional components 女, which is just a hint at the meaning for disambiguation purposes, and 良 (liáng), which is a hint at the pronunciation. Similar for 妮 (nī) with sound component 尼 (ní) and a similar historical meaning and development to the English word "lass". In the second and third cases the 女 is a semantic functional component, but it's still a different character with a different (if similar) meaning.

re: What I said about Kanji; I don't speak the language so perhaps I misunderstand how things work. My impression was that the mapping from character to meaning was looser in Japanese than it is in Chinese.