Please what are the approximate Width in Pixels of various Screens. Want to use it in Media Q.

Answer 1

bootstrap has 3 major sizes breakpoint:
768px
992px
1200px

These are not random. It is based on different devices and research.
Using these points are good.

But of course you can use as per your layout too.

Answer 2

//W3schools has perfect example for it
https://www.w3schools.com/css/css3_mediaqueries_ex.asp