There are two ways to read Chinese characters in Japanese, one is phonetic notation, that is, the pronunciation of this Chinese character when it was introduced into Japan from China, and the other is training reading, that is, the pronunciation of this Chinese character in Japanese. For example, Nanzi's pronunciation is なん, which is no different from Chinese, while its training is みなみ, which is completely different from Chinese.

As for when to read with training and when to read with pronunciation, it is not clear in one or two sentences. It is suggested to increase listening and reading. After a long time, you will naturally know which word should be pronounced when and how.