No, nobody understands the app because it is simply impossible to hold in your head 5-10 years worth of development by 100+ people across a complex product with complicated use cases and customer need (unless you were there the whole time and you are a very hard worker)

By the time you create the correct abstractions, customer needs and the market will have changed and thus your abstractions are now incorrect

It's both.

Sure, if you have project like facebook then you problably won't get away from complexity, and you NEED a tool to help you manage that.

But at the same time, if you assume you need such tool from the beginning, chances are you will over-complicate your solution and it will become a self fulfilling prophecy of trying to tackle complexity by adding more complexity