I am building a multi-store, single-user e-commerce platform using the MERN stack with TypeScript. Below is a snippet of my backend directory structure:

src
├───product
│   ├───controllers
│   ├───middleware
│   ├───routes
│   └───services
├───order
│   ├───controllers
│   ├───middleware
│   ├───routes
│   └───services

I want to make the backend more modular and maintainable, especially for product and order management, as they will grow in complexity with features like AI recommendations and predictions.

Question:
What design principles or patterns can I follow to achieve better modularity and maintainability in this context?