What is the term for studying all parts of a text in order to understand it better?

The term you're looking for is "textual analysis." Textual analysis is the process of studying a text in order to understand its meaning, structure, and context. It involves examining all aspects of the text, including the author's purpose, the audience, the genre, the style, and the language. Textual analysis can be used to interpret a variety of texts, including literary works, non-literary texts, and even visual texts.