Basically, you should aim to minimise complexity in software design, but importantly, complexity is defined as "how difficult is it to make changes to it". "How difficult" is largely determined by the amount of cognitive load necessary to understand it.
But I'm not experienced enough to tell, whether it is my inexperience that causes the difficulty, or it is indeed that the unnecessary complexity tha causes it.
With that said , get good at jumping into definitions, finding all implementations, then jumping back where you were. With a good IDE you can do that at the speed of thought (in IntelliJ that’s Cmb+b, Cmd+Alt+b, Cmd+[ on Mac). I only open more than one file at the same time when comparing them. Otherwise it’s much easier to jump around back and forth (you can even open another function inline if you just want to take a Quick Look, it’s Alt+Space). Don’t use the mouse to do that, things you do all the time can be made an order of magnitude faster via shortcuts. Too many developers I see struggle with that and are embarrassingly slow moving around the code base!
This is why I want to get into system programming. At least less business logic. They are still going to be complicated, but I feel it's a lot nicer to, e.g. go from the bottom of the process struct than the bottom of some half-ass business stakeholder!