How to do deep software refactoring if you can't stop development?

Question

Refactoring

The application has been under development for more than N number of years. They are already being used. Development takes 2 weeks, at the end of which there is a demo in front of the customer - we show what has been done. And there's nothing to show. And in the code already porridge.
What to do?
Where is the information on how to properly develop software so that this does not happen?
How to get out of this problem?

Answer 1

Complexity, intricacy of the code -> one of the natural consequences of the development of the project.
How to keep complexity within reasonable limits - this is what all these OOP, SOLID, microservice architecture and so on are about.
But the essence is the same everywhere -> divide one confusing code into independent parts (functional blocks, modules, microservices), which are easier to understand, easier to develop, reuse, remake, or even replace with other modules.