GCP

is one of the providers for serverless computing, offering different services like Machine Learning, Storage, Databases, Platform Engines etc. The idea of serverless computing is to prevent developers from managing servers themselves, instead they can focus on writing code and pay for only what they used to run their services.

Firebase

is platform developed by Google that helps developers create mobile and web applications.

Firebase

allows developers to have access to some Google Cloud Platform services like

• Cloud Firestore
• Cloud Functions
• Cloud Storage etc

Node

is a Javascript server-side programming language that allows developers build backend functionality for web/mobile apps quickly.

Express

is a simple web application framework that allows you manage your application from the server side. So anything that has to do with routing, views, handling requests etc.

MiddleWare

This is any number of functions that can be invoked inbetween your request handlers and the request made.

CORS ( Cross Origin Resource Sharing) This is a mechanism that allows or restricts requested resources on a web server depending on where the HTTP request is initiated from

For example, while you are still in the development stage - 
if you are using a frontend library such as React, your front end application
will be served on http://localhost:3000. Meanwhile, 
your Express server might be running on a different port such as 
http://localhost:2020.

Because of this, you'll need to allow CORS between those servers.

Next is used to pass control to the next middleware function. If not the request will be left hanging or open.