Mobile view affects Desktop view, that's not intended

I would like to style elements differently depending on the screen size.

Currently when I duplicate and rename class names from the Desktop view to the Mobile view; the further changes I make affect the Desktop view as well.

How do I stop this from happening?

Here’s an example of the desktop view.

Here’s an example of the mobile view

And when I make changes to the mobile view this is what it looks like when I remove ‘View Video’:

And this is what happens to the desktop view, the ‘View Video’ button is also missing:

Here is my site Read-Only: LINK
(how to share your site Read-Only link)