General approach to styling themes?

I’m currently working with Divi, but I’m looking for general advice on adding/modifying styles in any theme.

You can use developers tools to analyze a page and then do things like: copy and modify those styles using a child theme; or write declarations directly inside a theme’s (Divi’s) various”modules” (is this “inline” code?); or create new classes and add them to a theme’s (Divi’s) various modules.

How do you know when to do what? What’s best practice?

How can I figure out where’s the best place to place my styling?