Don't understand Desktop to Mobile sizing.

What I'm reading is SS automatically resizes to fit Mobile, but it doesn't with me. 

"We do this with responsive design, where your content automatically resizes based on the size of the browser window viewing your site. "

Posted this in pages forum too. Unsure where too post. Getting lost in the help guides. For a layman, sorry just don't; find the guides user friendly. 

Anyhoo. I have one landing page with graphics. I have removed the header and footer. the landing page is 4 x 2 second videos. I just want the exact same thing on mobile landing page. But it's severely cropped. Just the middle of desktop, which is about one third of original. Playing around with the edit tool doesn't;t solve as well. That's even more confusing. 

Anyhoo hoo.. How do I do this?? Cheers

Watching this video and it shows SS just auto re-sizing with a little crop on the sides. But that doesn'thappen with me. https://support.squarespace.com/hc/en-us/articles/205815398

Can you share your website URL? It's important to know whether you're using the latest editor which requires you to edit the mobile layout manually in order to answer your question.