Another way to say what Kateryna S. mentioned is that the viewport renders differently depending on the size of your device. For example let's say you made a responsive web app using material design that through the grid system looks one way on your desktop browser and one way on your mobile device. The Mobile device has a much smaller viewport and will render the site using the smaller grid.
For example, if you look at material design and resize your browser you will see how it will look on different viewports. If you resize your browser to the smallest size that mimic basically what you would see on your mobile device.
https://material.io/design/components/cards.html#usage
The window is just your browser view and is not related to other devices.