Mobile view on desktop

Hi
How could I see the mobile version of my website on desktop ?
When I tried to do it through the admin panel of the theme it is not
really shows the mobile version. On my desktop version I have a video
and on mobile the video is not loading but an image.
The theme mobile icon shows video on the mobile icon so it is not good
emulator.

I also tried Chrome extension and other emulators but they also not really work
as mobiles. They only detects the screen size but not other factors.
For example, the official theme site: https://2017.wordpress.net:
On desktop with mobile view of Chrome developers, the top header looks fine
but when I open it on mobile, the header image gets larged rolling down and shrinkied
when back up.

So…what is the code needs to tell the browser I am on a mobile browser
while I am on desktop ? In other framework I add “&layoutType=mobile” to the URL.