It's specifically when I click on the demo workouts link in your post - when I click the main link to your site, everything seems to work fine (although I haven't finished the onboarding process).

When I click on that on my phone (Moto G5+, Firefox), most of the page is cut off, and I need to scroll around to see anything. It's like the page is designed for a device about twice the size of my one, and my browser has zoomed into the upper-left corner.

I'm on my phone so I can't explore further, but my guess is that the viewport settings aren't right for that page specifically, or there's some element with a min-width that's making the page wider than it needs to be. I get the same effect on mobile Chrome so I assume it's more about the display size then the browser.