How do some websites make pc sites different from mobile ?

Question

On w3schools.com they display all the content of the site on the left of the website and when you make the window smaller it changes into a button you have to click to see a scroll down of the site content . I'm just curious to know how they do it
You can see for yourself :
https://www.w3schools.com/css/

Answer 1

Did you ask there?

Answer 2

Responsive Web design

https://www.sololearn.com/Course/Responsive-Web-Design/?ref=app

Answer 3

Richard
1) Create a boolean variable.
2) Verify the size of the screen.
3) If the screen/window is less than or equal to the dimensions of a mobile device make the variable as true.
4) Have two different designs for both PC and mobile devices in HTML.
5) Based on the value of boolean show/hide the specific designs for PC / mobile devices.