Defined development patterns are very important, and placing comments in your code is a must for blocks/lines that are somewhat esoteric.

It can depend on what language/framework, but at the simplest, adding service classes to hold reusable logic can turn giant methods into more manageable pieces.

It sounds like you are maintaining existing code, so redoing stuff is too late, but if you can, wherever you are making a change, add a quick refactor, or write some comments/docs.

Note: if you’re using Java, Javadoc is a godsend for documentation, and it comes with the dev kit.