Different text mobile vs desktop

Hi everyone, I have an issue in regard of the text, I want to have different text on mobile vs desktop.
As in “xyz” desktop and “x” on mobile, a shorter (self written) version of the same content within boxes.
I couldn’t figure out a way of doing it, is there anything less complicated (and causing the site to being slower) than coding it?

Hope someone can help and knows a simple way about it!

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

I am having the same issue. I can custom style everything about a text box for each device EXCEPT the behavior of the text within the text box itself. If I change the text in the text box in any way, it is changed on every device. How can I change the text copy for each device while maintaining the same text box style for each device?