It’s not a junior developer’s place to refactor a codebase for maintainability. That’s for high paid consultants like Joost Visser’s SIG.
Seriously, though, you should read Joost Visser’s Building Maintainable Software. Read the Java edition if you use Java or if you can’t find an edition for your language.
It’ll help you prioritize things. For example, you should probably let 20-line functions slide. But that 200-line function is a problem waiting to hit at the worst time.