German Layout Magic Keyboard - '@' doesn't function

On German Standard, @ is located at Option Q (not Option L). Try that. I think the latest Apple German keyboards should even have @ printed on the Q key maybe.

If you set your input source to German instead of German Standard, then @ should be on Option L.

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[Edited by Moderator]

"German Standard" does have an extra German character you don't find on "German:" The uppercase ß, or ẞ, which is on Option G.

System Settings -> Keyboard -> Shortcuts -> Typing And define the "Alternative Character Key" I used Right Ctrl Key.

Combine this key with Q or 2.

Result: @