I could not find any code example on "How do two adjacent logical layers interact in a layered or n-tier architecture pattern?" I found that model and view is included in controller for them to work as a whole (just to make it clear, I am not talking about MVC here, but layered or n-tier architecture pattern overall).

However, I think we have to use interfaces to loosely couple each layer. How does it work?

Is there any way I can deploy each layer separately. In other words decouple them completely so that I can run tests on each layer's input and output boundary.