Answer 1

Hi Amin
a PWA is a JavaScript HTML CSS app that uses new web technologies to give a Native Experience, especially on mobile.
These include but not limited to
: Service Workers that offer the possibility of offline use of the App
: Asynchronous loading to not have code block the main event loop
: Reactive technologies like React Native for Server side rendering using Shadow DOM
: New web components that run at near native speeds for Single page Apps
: Install to home page, to offer easy launch and notifications

Answer 2

You need web app manifest
https://developers.google.com/web/fundamentals/app-install-banners/
and you need to clear the hurdles
https://infrequently.org/2016/09/what-exactly-makes-something-a-progressive-web-app/

Answer 3

And here are some demos of PWAs you can install on your phone
https://pwa.rocks/
And Tutorial
https://codelabs.developers.google.com/codelabs/your-first-pwapp/#0