How do you make website layouts that work for both desktop monitors

You're are looking for "responsive" design. Check out Bootstrap, it'll do most of the work for you.

>>58060347
Im not gonna use Bootstrap before i have better knowledge of css and js

Just look up "responsive design" and copy the things that other people do.

>how the fuck do i determine the screen size
You don't, necessarily. You determine the browser through useragent. Mobile browser software = mobile device. Width in pixels also works. My desktop monitor is wider than it is tall. If I shrink my browser width to, say, 960px, some websites do indeed switch to mobile layout.

>>58060333
Pajeet pls go, you can google that information in less than five minutes.

But I'll help you anyway. An easy way to do this is to use a CSS framework like Bootstrap, Materialize or Pure. They all come with some sort of a grid system that'll accommodate to the user's screen size if you have enough brains to follow the rules. Should you want to go deeper, try the CSS Flexbox feature, here's a guide for that https://css-tricks.com/snippets/css/a-guide-to-flexbox/

The pro of Flexbox is that it requires no external libraries, but it only works on modern browsers unless you include a polyfill (library that implements the feature in older browsers).

Bootstrap rows and cols, literally all you need to make your shitty website responsive

>>58060347
>>58060362
>>58060403
>>58060615
>all these bootstrap babbys

phone browsers pretend to have a different screen size so that websites look ok given the physical display size

look up media queries and "meta viewport"