Browser desktop mode not really the same as PC desktop
There are various ways of creating responsive webpages.

the developer can ask the browser to tell whether it is windows os or android or iPhone or which browser it is etc.

Secondly, most often responsive design is implemented based on the width available to the browser window. To learn more you can search the web or try for yourself as I explain below

In fireFox browser on PC go to menu -> Web Developer -> Toggle tools or press Ctrl + Shift + I



Now click on the button on left of three dots. You can change the size of page to see how it looks on browsers of different sizes.



Hope this helps.