What does `morgan` module have to do with express apps?

Morgan is used for logging request details. However, the snippet in your question doesn't make sense because it's not actually a single coherent snippet top to bottom. It is a series of examples of the various types of options you can pass to morgan. In a real program, you would only need one of them. For example:

var express = require('express')
var morgan  = require('morgan')

var app = express()
//This tells express to log via morgan
//and morgan to log in the "combined" pre-defined format
app.use(morgan('combined'))
//That's it. Everything in your snippet after this are just
//other variations your might want to use

Morgan is basically a logger, on any requests being made,it generates logs automatically.

Morgan is a popular HTTP request middleware logger for Node.js and basically used as a logger. It can be used with node js' winston package to consolidate HTTP request data logs with other information.

Morgan: is another HTTP request logger middleware for Node.js. It simplifies the process of logging requests to your application. You might think of Morgan as a helper that collects logs from your server, such as your request logs. It saves developers time because they don’t have to manually create common logs. It standardizes and automatically creates request logs.

Morgan can operate standalone, but commonly it’s used in combination with Winston. Winston is able to transport logs to an external location, or query them when analyzing a problem.