What patterns can help me organize big project with modular structure?

For example, I have a project with ~500 pages and a domain with big variety of different functions. Also there are requirements about modular structure of the app. It should be not so hard to turn off and turh on some of the features. There is caviet that features can integrate with each other. Example: You can create different documents in the app (bills). Also you can create payments which pay those bills. Also you can you can bind payment with bill while creating a payment and vice versa. There are cases when you do not have access to documents functionality, only payments. So, you cannot bind payments with documents and do not see documents in UI. What if I have ~30 such modules with different interconnections. How can I organize the app so I can turn on and off any of modules?