setImmediate() and process.nextTick() are two functions which allows us to control the order of execution of our code in the event loop. Both of these functions schedule our callback handlers in the event queue. But they are executed at different phases of the event loop.
Code to an interface is a technique used in programming where you build your application based on abstractions(interface) and not concretions(classes).
Composer is an application-level dependency manager for PHP. Dependency simply means libraries/packages your application depends upon. Apart from managing the dependencies, composer will also autoload the files needed for the application.