What is an Express Server?
Express is a node js web application framework for building web and mobile applications. It is used to build a single page, multipage, and hybrid web application. It's a layer built on top of the Node js that helps manage servers and routes.

It also provides a command-line interface tool (CLI) called Node Package Manager (NPM), where developers can source for developed packages. It also forces developers to follow the Don’t Repeat Yourself (DRY) principle.

The DRY principle is aimed at reducing the repetition of software patterns, replacing it with abstractions, or using data normalizations to avoid redundancy.

Why use Express Js?
Was created to make APIs and web applications with ease.
It saves a lot of coding time almost by half and still makes web and mobile applications are efficient.

• High performance
• Fast
• Unopinionated
• Lightweight

Middle Ware
Middleware is a request handler that has access to the application's request-response cycle.

-Routing
It refers to how an application's endpoint's URLs respond to client requests.

Templating
Creates a html template file with less code and render HTML Pages

Debugging
Express makes it easier as it identifies the exact part where bugs are

Advantages of using Express with Node.js?

• Express is Unopinionated, and we can customize it.
• For request handling, we can use Middleware.
• A single language is used for frontend and backend development.
• Express is fast to link it with databases like MySQL, MongoDB, etc.
• ​​Express allows dynamic rendering of HTML Pages based on passing arguments to templates. It is the backend part of something known as the MEVN stack.

The MEVN is a free and open-source JavaScript software stack for building dynamic websites and web applications that has the following components:

MongoDB: MongoDB is the standard NoSQL database
Express.js: The default web applications framework for building web apps
Vue.js: The JavaScript progressive framework used for building front-end web applications
Node.js: JavaScript engine used for scalable server-side and networking applications.

What is it used for?
Express.js is used for a wide range of things in the JavaScript/Node.js ecosystem — you can develop applications, API endpoints, routing systems, and frameworks with it.

Single-Page Applications
Real-Time Collaboration Tools
Streaming Applications
Fintech Applications

Limitations of Express Js
Sometimes, there is no structural way to organize things, and the code becomes non-understandable.

• There are so many issues with callbacks.
• The error messages that will come are challenging to understand.

Companies That Are Using Express JS

•  Netflix 
•  IBM 
•  ebay
•  Uber

Sources
https://kinsta.com/knowledgebase/what-is-express-js/#fintech-applications
https://youtu.be/0QRFOsrBtXw?si=emIWngL4bcR7hVeD