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Understanding that you don't really need to understand it all and you need to understand only part of them. If you do try otherwise, you run into overhours :-)

Most popular technique to 'just do the thing' is using 'if my case do #1, else do #2'. Mostly because nobody really understand the business logic. Many reasons, no real one single document with spec, only wikipedia called confluence where you have half truths, barely truths and contradicting each other statements.

In such situation you almost never will understand the logic because there are even no use cases and thus all you can get are regression tests (because there are no use cases list). It gets accumulated over years, many externals / consultants with half-life time 1 year and so continues - chaos in specs (sometimes even transfering from one tool to another one with losing info) and big rotation in companies too dependent on externals.

So instead doing simple if-else, it's better to use wrapper / decorator or even better proxy of chain if more complicated.

It all depends and this textbox is too short to explain the fun facts :-)