Some ways of reading kanji are derived from Chinese, which is where the だん in だんせい comes from. Other ways of reading kanji are based on the Japanese words that already existed when Chinese writing arrived in Japan. That's おとこ.
Jisho lists these on the detail pages for the kanji characters. Chinese-derived readings are called "onyomi". Japanese readings are called "kunyomi".
Here's an article that explains the difference and gives some rules of thumb, though there are always lots of exceptions.
Read word for word, not character for character. That's what the brain is doing anyway even in other languages, so it shouldn't really pose a big problem.
Pronunciation when reading Man or Boy
I’m a new learner and I noticed when reading 男 it is pronounced differently when it’s used with 男性 (Man) then compared to when reading 男の子 (Boy) why is 男 pronounces differently and I’m assuming this happened often in kanji and how would I know when the pronunciation changes other then reading ahead or is predicting the word the only way.