How to write code that it will take us from signing up page to the inside of the app in the form of website?

Question

I am learning HTML to make a website for an app
My question is after signing up or loging in how do you write what will be the next
To make it more understandable let me give an example
Facebook is an app
There's also a website for Facebook
At first you have to write HTML code to display the signing up page, after signing up or loging in
We are taken to the inside of the app in the form of website
So how to write code that it will take us from signing up page to the inside of the app or do I have to learn other programming language for that?

Answer 1

It is better to do via back-end like PHP, or Ajax, it is more secure then.
Or you can made second page and paste it inside your sign-in page with display:none; and tben use display:auto;