Which web browser view should I consider for further work on phone compatible CSS?

(https://i.stack.imgur.com/xCBJz.jpg) (https://i.stack.imgur.com/sQeW3.jpg) Which of these web browser views should I consider for further work on phone compatible CSS? Non-device mode in Chrome isn't showing up CSS of below 800 and 600 pixels, I have no idea why. I had earlier wrote phone CSS for other pages, which was detectable on the phone after upload to the server, but the code isn't just getting detected.