ø this symbol instead of the at sign

"iOS" is a special indication written on this XtremeMac keyboard

"iOS" is written in blue at the bottom right of the A key
(just like "Android" is written below the Z key, and "Windows" below the E key)

So to activate the Mac configuration you need to press " Fn + A " (e.g. " Fn + iOS ")

The manual, on p.2, where it is written: https://xtrememac.com/wp-content/uploads/2019/10/XM-KEY-BT4_UserGuide.pdf

 "iOS" is a special indication written on this XtremeMac keyboard 

"iOS" is written in blue at the bottom right of the A key 
(in the same way that "Android" is written under the Z key, and "Windows" is under the E key) 

So to activate the Mac configuration, you need to press " Fn + A " (e.g. " Fn + iOS ") 

The manual, on p.2, where it is written: https://xtrememac.com/wp-content/uploads/2019/10/XM-KEY-BT4_UserGuide.pdf

???

iOS has absolutely nothing to do with a Mac computer.
iOS only concerns the iPhone and iPad.
For Mac, it is Mac OS or macOS, depending on the version in question.

Sincerely.

And so, what is the selected key layout in the "Input" menu? And still nothing in the keyboard viewer when you press the Option and/or Shift keys? This seems impossible. If necessary, take a screenshot (combination Command Shift 4) so we can fully understand the situation...