Responsive website

Question

What is the logic behind Responsive websites?
how they know is user is using phone or any other device?

Answer 1

In css, you can use media queries to check for the width, height and resolution of the target device, and set your styling rules differently depending on different sizes that you chose.

Answer 2

1st main thing is your browser resolution,
2nd what usually serverside will see is a useragent
3rd mostly used in front end javascript is a Navigator