Where are required modules visible?

I have a problem understanding NodeJS require(). Basically I have this 3 files:

moongooseconfig.js

var config = require('./config');
var mongoose = require('mongoose');

module.exports = function() {
    var db = mongoose.connect(config.db);
     require('../app/models/user.server.model');

    return db;
}

usercontroller.js

var User = require('mongoose').model('User');

    exports.create = function(req, res, next) {
        var user = new User(req.body);
        user.save(function(err) {
            if(err) {
                return next(err);
            } else {
                res.json(user);
            }
        });
    };

And the server.js

process.env.NODE_ENV = process.env.NODE_ENV || 'development';
var mongoose = require('./config/mongooseconfig');
var express = require('./config/express');


var db = mongoose();
var app = express();
app.listen(3000);

My understanding of require was that the required module is only visible in the JS file which required the module. I do not get why the user controller can use the mongoose Model 'User' without requiring the model file. Yet the mongooseconfig requires the model file inside a function without saving it to a variable.

Can somebody tell me what happens there? Can every file access modules once they where required anywhere? (Maybe I 'm just to blind, but I can not find the answer in the Node docs and googling "nodejs require scope" does not gave me any good results)