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
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