Responsive design, applying Js and functions with validation bnt

I would like to learn more about, how can I get more control with the screen size, because I felt that I worked only seeing and trying to follow the instructions, but always o doubt if the mesurements that I'm using as the complete screen are fine, even because when I saw the result on my screen show me that everything is Ok, but once I submitted the challenge, the size changed, the buttons, everything, even the rectangule now appears a square xD. I used the programmer panel and I selected the 1440 px screen to do the pocess so I am confused why didn't work